Preamp for Analog Discovery 2

[David Hess]

The differential configuration assuming you drive two channels of the AD2 raises the AD2's input noise by Sqrt(2).  Differential amplifiers like the 7A13 and 7A22 have more noise even when one input is grounded compared to single ended amplifiers.

Of course the same thing applies when two single ended channels are used in add and invert mode except if this is done digitally, then the quantization noise of the separate channels *also* adds together.

[xani]

Ok so Xani I should be looking at the figure showing approx. 900uVrms (5.8mVpp) noise, I suppose this corresponds to the "high gain mode" of the AD2. This noise results in SNRFS (5Vpp full-scale sin signal) of approx. 65.86dB -> ENOB of 10.6 bits. (By the way your results are similar to these: https://forum.digilentinc.com/topic/4306-analog-discovery-2-wavegen-noise-below-50mv/)

Also note that both channels are shifted slightly from zero, and by different amounts.

As for design, you can do something clever like auto-switching gain of input amplifier via software using digital IO pins. I've seen some clever scripts written for it, like THD meter that just scripted built-in instruments to graph THD/frequency

[thanasisk]

The differential configuration assuming you drive two channels of the AD2 raises the AD2's input noise by Sqrt(2).  Differential amplifiers like the 7A13 and 7A22 have more noise even when one input is grounded compared to single ended amplifiers.

Of course the same thing applies when two single ended channels are used in add and invert mode except if this is done digitally, then the quantization noise of the separate channels *also* adds together.

The differential output is used to drive a single differential channel of the AD2.

I am trying to understand the operation of the circuit of p. 9 (fig. on the top left) of the datasheet. There are two diff-in single-ended-out LT1102 amplifiers connected in (inverted) parallel mode. The positive output is the normal signal amplified by the top amplifier (with G=10 or 100) plus the output referred voltage noise of the top amplifier. The negative output is the inverted signal amplified by the bottom amplifier (with G=10 or 100) plus the inverted output referred voltage noise of the bottom amplifier. So the diff output is G*Vi+G*N1-G*(-Vi)-G*N2 = 2*G*Vi + G*(N1+N2). The output referred voltage noise is then INCREASED by sqrt(2) and the voltage gain is G=20..200 depending on the config of the top&bottom LT1102s.

By the way, the diff output configuration in the LT1102 data sheet p.9 is very similar to p. 35 of http://www.thatcorp.com/datashts/More_Analog_Secrets.pdf

There it also says that a "low-Z attenuator" between the (differential) output and the ADC input is required: "optimizes noise & headroom". Is  this statement importance for our case (whereas the AD2 input attenuator is high-Z)?

Edit: corrected the output noise conclusion for the parallel inverted case..

[thanasisk]

Ok so Xani I should be looking at the figure showing approx. 900uVrms (5.8mVpp) noise, I suppose this corresponds to the "high gain mode" of the AD2. This noise results in SNRFS (5Vpp full-scale sin signal) of approx. 65.86dB -> ENOB of 10.6 bits. (By the way your results are similar to these: https://forum.digilentinc.com/topic/4306-analog-discovery-2-wavegen-noise-below-50mv/)

Also note that both channels are shifted slightly from zero, and by different amounts.

As for design, you can do something clever like auto-switching gain of input amplifier via software using digital IO pins. I've seen some clever scripts written for it, like THD meter that just scripted built-in instruments to graph THD/frequency

This "DC-RMS" measurement, this is the DC offset of the channels right? I suppose this reduces the dynamic range so in my calculations for the SNR in the 500mV/div "high-gain" range of the AD2, I should take the maximum input signal as 5Vpp*0.3535 (=Vrms)  minus this DC-RMS offset?

Yes indeed a switching system with e.g. relays using the digital IO pins is a cool idea! I believe it is used with the impedance analyzer board.

[David Hess]

The differential configuration assuming you drive two channels of the AD2 raises the AD2's input noise by Sqrt(2).  Differential amplifiers like the 7A13 and 7A22 have more noise even when one input is grounded compared to single ended amplifiers.

Of course the same thing applies when two single ended channels are used in add and invert mode except if this is done digitally, then the quantization noise of the separate channels *also* adds together.

The differential output is used to drive a single differential channel of the AD2.

I am trying to understand the operation of the circuit of p. 9 (fig. on the top left) of the datasheet. There are two diff-in single-ended-out LT1102 amplifiers connected in (inverted) parallel mode. The positive output is the normal signal amplified by the top amplifier (with G=10 or 100) plus the output referred voltage noise of the top amplifier. The negative output is the inverted signal amplified by the bottom amplifier (with G=10 or 100) plus the inverted output referred voltage noise of the bottom amplifier. So the diff output is G*Vi+G*N1-G*(-Vi)-G*N2 = 2*G*Vi + G*(N1+N2). The output referred voltage noise is then reduced by sqrt(2) and the voltage gain is G=20..200 depending on the config of the top&bottom LT1102s.

Isn't my analysis correct?

I do not think that is strictly correct.

The output voltage is doubled but the noise in each amplifier is uncorrelated with the other so it adds as well when seen at the output; it is like having the two noise sources in series resulting in Sqrt(2) higher noise at the output.  This is still an improvement in signal to noise ratio since the signal gain is even higher.

But compared to a single ended amplifier, the differential configuration has more noise and a lower signal to noise ratio.

By the way, the diff output configuration in the LT1102 data sheet p.9 is very similar to p. 35 of http://www.thatcorp.com/datashts/More_Analog_Secrets.pdf

That is a nice reference from THAT Corp.  It is the same circuit.  Using a pair of instrumentation amplifiers like that has the advantage of providing high impedance inputs.  "Op Amp Applications Handbook" by Walter Jung from Analog devices gives some examples of similar circuits starting on page 480 and 520.

There it also says that a "low-Z attenuator" between the (differential) output and the ADC input is required: "optimizes noise & headroom". Is  this statement importance for our case (whereas the AD2 input attenuator is high-Z)?

It has nothing to do with the input attenuation.  The point they are making is that if the amplifier's output noise is significantly higher than the input noise of the next stage (ADC), then the excess gain has little benefit and only serves to lower the dynamic range.  Radio receivers face this problem all the time.

[blackdog]

Hi,

There are many ways to build a preamp and i want to show one of my design.
It is doing 60dB, has reasonable low noise and a good puls response.
More than 6MHz -3dB point, about 4MHz -1dB and 900KHz -0.1dB.
Square wave has no overshoot :-)   (If you build the circuit correctly)




The amplifier has input and output protection and can work on 2x9V battery or if you need more output on a mains power supply, but watch for the power dissipation of the THS3061!
I hackt the THS3061 with a heathsink and some thermic glue to get a higer output when working on a 2x12V power suppply.
The square-wave response is without abberations is only possible if the connections are kept short!
And maybe adjust the capacitor C25 a little.

Build to work directly connected to a D.U.T or via a scope probe.
The output is designed to drive a NOT terminated schort peace of coax cable (Like a RG58 BNC patch kabel not more than 3 feet)

Dutch website with explaning and picture, Google translate is your friend.
https://www.circuitsonline.net/forum/view/135023#highlight=meetversterker


PS,
No, you can't replace the opamps and then expect that the circuit will give the same performance., changing something and you are on your own :-)

Kind regarts,
Bram

[thanasisk]

Oh yes, of course the uncorrelated noise adds up! Thank you for the acute observation and also the very interesting reference David. I have updated my nose calculations in my posts above. The output noise for a differential output with two LT1102s in parallel inverted mode is indeed quite higher while I suppose the performance gains achieved (CMR rejection,..) compared to a single ended solution would not pay off (we will not have to drive any significant cable/pcb trace length at the output anyway.. ).  Also given the price of one LT1102, makes me wonder whether it is better to drop the differentential output requirement altogether or just use a secondary cascaded stage with e.g.THAT1606 or similar (have to
do the noise calculations for a cascade as well..)

Bram thanx for the interesting design, I found it before when I was researching.  My requirements for this design is high input impedance  with differential inputs (at least at this stage)..

[thanasisk]

As a side note, I was amazed at the scarcity of high input impedance amps offered out there.. Makes design life easier i guess by not having ample choices 

[blackdog]

Hi thanasisk, :-)

Misschien zoek je naar de verkeerde de opamp?    (Maybe you are looking for the wrong the opamp?)

The problem is that the fast opamps almost all have a low input impedance, but you were already familiar with that.
There is a beautiful TI IC that has a Fet input and that might have a low enough noise number for you.
The gain is 2x of this IC and you can then use fast BJT opamps behind for your gain of 40 or 60dB.
You will have to calculate if the 1/F noise is low enough for you.
Maybe more than one parallel if you just don't get your noise figure...
http://www.ti.com/product/opa653/description?keyMatch=opa653&tisearch=Search-EN-Everything

Kind regards,
Bram

[xani]

Also given the price of one LT1102, makes me wonder whether it is better to drop the differentential output requirement altogether or just use a secondary cascaded stage with e.g.THAT1606 or similar (have to
do the noise calculations for a cascade as well..)

AD6066, which is used in AD2, looks pretty interesting (FET, low noise), but probably bit too slow to get to the full AD2 speed with any significant gain. But it is a bit cheaper than LT1102.

[thanasisk]

Both AD8066 and OPA653 are interesting parts but my understanding is that they were specced for high speeds&bandwidth at the expense of higher noise in low frequencies (and I do want decent  DC-22KHz performance..) .  Also I would be reluctant to consider them as they do not seem to be very forgiving to a diyer in terms of pcb design and parasitics.. 

[blackdog]

Hi thanasisk,

Really very low noise such as for a good dynamic microphone amplifier or a band microphone is something quite different from a low noise amplifier with an input impedance of 1Meg.
You have to decide what you want, whether you want it or not. 

You can build an amplifier with a Fet input like with the 2SK170 and its family,
but it will always have a large input capacity and the bandwidth of that amplifier will depend on your steering-impedance.
The latest version of The Art Of Electronics contains a few amplifiers with graphs of the bandwidth to be achieved with respect to the input impedance.

To measure low noise of power supplies and voltage references, you do not need a 1Meg input impedance, 10 to 20K is usually sufficient.
It usually comes down to the fact that you will need more than one type of preamplifier for your LAB.

Then again, don't just think of amplifiers, but also of filters!
Often it is useful to have some filters available after the first amplifier stage so that e.g. hum can be removed via a notch filter without overloading your measuring device.
A second handy filter is 400Hz High Pass to filter everything below that frequency.
Think of 12 to 18dB per octave for the filters and 50dB for the notch.

Kind regards,

Bram

[thanasisk]

The main idea behind this design was to extend the capabilities (sensitivity) of the AD2 without altering its "character" (1M input impedance and differential inputs).

Very low noise low input impedance preamps are very tempting  but a high input impedance is really what I need for also being able to support a 10x passive probe at the input.

Yes indeed adding filters are important (notch,HPF but LPF as well) and I will do consider adding them later in the design process..

[DaJMasta]

I still think high input impedance is a near trivial addition if you just have a low gain buffering stage.  Say you want 10MHz of bandwidth, so you find a high input impedance amp (maybe start looking at < 10nA input bias current) and a low noise figure.... but instead of looking the perfect frontend amp with 500MHz+ GBP (and a sky high price) to do all your gain in that stage... you look for 50MHz GBP or less.  Then your input amp is very high impedance, low noise, and offers some gain, maybe you find one with 30MHz GBP so you use a fixed gain of 3, which costs a tenth or less of the fancy high performance amp and offers lower noise.  The output of that amp then drives enough current to be able to power almost any amp for your primary gain stage, and your secondary amp can be higher noise as well - the noise floor will be at least the input amp's noise times its gain, so in this example, your second stage amp for high gain can be up to 3x the noise figure before it starts dominating the noise of the pair.

I don't know if you care about selective filtering, but it is a good idea to put in basic low pass filters into your amp stages, because then any additional GBP over what is needed (in the spec or beyond) doesn't amplify signals that are outside of your specified bandwidth and contribute to your overall noise.  This can be as simple as an LC filter in the path between the two amps (and maybe another after the second stage) or a good dielectric cap in parallel with the feedback resistor if you're using an inverting amplifier configuration - minimal parts addition and design concern, but lower noise.

[thanasisk]

I still think high input impedance is a near trivial addition if you just have a low gain buffering stage.  Say you want 10MHz of bandwidth, so you find a high input impedance amp (maybe start looking at < 10nA input bias current) and a low noise figure.... but instead of looking the perfect frontend amp with 500MHz+ GBP (and a sky high price) to do all your gain in that stage... you look for 50MHz GBP or less.  Then your input amp is very high impedance, low noise, and offers some gain, maybe you find one with 30MHz GBP so you use a fixed gain of 3, which costs a tenth or less of the fancy high performance amp and offers lower noise.  The output of that amp then drives enough current to be able to power almost any amp for your primary gain stage, and your secondary amp can be higher noise as well - the noise floor will be at least the input amp's noise times its gain, so in this example, your second stage amp for high gain can be up to 3x the noise figure before it starts dominating the noise of the pair.

I don't know if you care about selective filtering, but it is a good idea to put in basic low pass filters into your amp stages, because then any additional GBP over what is needed (in the spec or beyond) doesn't amplify signals that are outside of your specified bandwidth and contribute to your overall noise.  This can be as simple as an LC filter in the path between the two amps (and maybe another after the second stage) or a good dielectric cap in parallel with the feedback resistor if you're using an inverting amplifier configuration - minimal parts addition and design concern, but lower noise.

Would you thus propose a differential op amp instead of an instrumentation amp? (see my requirements above).

An LPF I think will be I included and as you say can be configured with minimal part additions.

[David Hess]

Is there a specific reason you want to use two channels of the AD2 in a differential configuration?  Why not simplify things and convert the differential input to a single ended output?

Is fast overload recovery a requirement?  Shunt feedback amplifiers have problems with this.

It is not explicitly listed above but is common mode suppression a requirement?  This only applies if you want to use two channels of the AD2 as a differential input.

The dual LT1102 achieves this as shown in the datasheet and THAT's application note but it is not particularly low noise and the bandwidth at a gain of 10 (20 when using 2 as shown in the figure) is barely above 1 MHz; pay attention to the undistorted output versus frequency graph which shows the full power bandwidth at different output levels.  The alternative instrumentation amplifiers from AD, the AD8429 and AD8421 are not any better in regard to full power bandwidth with about the same slew rates.

Now the full power bandwidth can be increased by adding gain *after* the instrumentation amplifiers which is easy to do.  Doing so removes the limit of full power bandwidth discussed above from the instrumentation amplifiers so small signal bandwidth is again what matters and there the AD8429 and AD8421 are much better than the LT1102 which is stuck at a minimum gain of 10.

But if you use the AD8429 and AD8421, then a high input impedance buffer or preamplifier is required and here is where a pair of fast low noise JFET input operational amplifiers can serve.

[DaJMasta]

If you need the channel to be fully differential, you can always just duplicate the circuit - you add some variability to the system, but given the bandwidth you can probably just make sure your gain resistors are tight tolerance and your opamps have low input offset voltage and you'll probably be fine so long as the two are colocated on a board or in the same enclosure.  If you need a differential input, you can duplicate the first low gain stage and then send each into a secondary amp to move to single ended and do your final gain stage with that.  If you want differential input into the analog discovery for whatever reason but are fine with single ended inputs, you can just have an inverting follower amp after the output stage to get a mirrored signal (though I don't think it's going to promote signal integrity unless you need big time EMF rejection between the output of the preamp and the input of the analog discovery).

Personally, I don't know why you'd need a differential signal for this application, but perhaps your situation can demand it.  I would certainly prefer two distinct channels to play with and just go into a differential mode in software, and at these frequencies and signal levels, unless your area is very noisy in terms of EMF (and if it is, you probably need to shield the analog discovery too), single ended should be able to give you plenty of signal integrity and responsiveness within the bandwidth you're looking at and will be cheaper to implement (more choices of parts, cheaper options, fewer parts).

[xani]

The main idea behind this design was to extend the capabilities (sensitivity) of the AD2 without altering its "character" (1M input impedance and differential inputs).

Very low noise low input impedance preamps are very tempting  but a high input impedance is really what I need for also being able to support a 10x passive probe at the input.

Why would you want to measure very low signals using 10:1 probe ?

There is always an option of having a bunch of switches/relays and just having 2 switchable input amplifiers with different characteristics but that would complicate it a bit.

[thanasisk]

Been busy with simulating the whole frontend up to the ADC driver (used TINA-TI with AD8065 pspice model from AD). I just wanted to have a more precise understanding of the noise and frequency performance of the AD2 before discussing more about the requirements; I will thus get to your latest questions in a bit.

BW is 31.5 MHz (so this is what the AD2 website means by "30 MHz+")  BUT  what they fail to disclose is that the  low frequency performance does suffer! See attached Bode diagram.

I will therefore need a slight DC-100KHz boost in the preamp to equalize that 1-1.5dB non-flat frequency response.

The input referred noise is higher than what you had calculated David.

[David Hess]

BW is 31.5 MHz (so this is what the AD2 website means by "30 MHz+")  BUT  what they fail to disclose is that the  low frequency performance does suffer! See attached Bode diagram.

I will therefore need a slight DC-100KHz boost in the preamp to equalize that 1-1.5dB non-flat frequency response.

To me that looks like it is caused by improper compensation of the input divider.  The corner frequency is about right and the AD2 only includes one compensation adjustment.  High impedance attenuators are greatly affected by the printed circuit board material which is not accounted for in a simulation.  Look up oscilloscope "hook".

The input referred noise is higher than what you had calculated David.

It was only a "good enough" estimate and I did not bother including 1/f noise.  I wanted to find the reasonably minimum gain to design for.  I am pleased it is as close as it was.

1/f noise is a big deal in DC precision applications and where most integrated oscilloscope front ends are terrible; it is difficult to control in a wide bandwidth input stage.  Dual path input amplifiers which use a divider before the DC amplifier are even worse (as the AD2 design shows) and make those old dual JFET in a totem-pole configuration used for input buffers look really good.  They even make the Tektronix 7A13 look good.

The Tektronix 7A13 is my reference for oscilloscope vertical amplifiers because it is so noisy.  But it isn't a noisy as a modern oscilloscope!  The 7A13 comes out as better than 20nV/SqrtHz.  Single ended JFET input amplifiers of that age are about 3nV/SqrtHz.  The 7A13 is much worse because it has differential inputs and all of the extra circuitry to support a +/-10 volt common mode input range without input dividers.

What you seem to want is a differential probe which supports using x10 passive probes.  I might have suggestions for an alternative design but it depends on one of the questions I asked:

Is there a specific reason you want to use two channels of the AD2 in a differential configuration?  Why not simplify things and convert the differential input to a single ended output?

[thanasisk]

BW is 31.5 MHz (so this is what the AD2 website means by "30 MHz+")  BUT  what they fail to disclose is that the  low frequency performance does suffer! See attached Bode diagram.

I will therefore need a slight DC-100KHz boost in the preamp to equalize that 1-1.5dB non-flat frequency response.

To me that looks like it is caused by improper compensation of the input divider.  The corner frequency is about right and the AD2 only includes one compensation adjustment.  High impedance attenuators are greatly affected by the printed circuit board material which is not accounted for in a simulation.  Look up oscilloscope "hook".

The input referred noise is higher than what you had calculated David.

It was only a "good enough" estimate and I did not bother including 1/f noise.  I wanted to find the reasonably minimum gain to design for.  I am pleased it is as close as it was.

1/f noise is a big deal in DC precision applications and where most integrated oscilloscope front ends are terrible; it is difficult to control in a wide bandwidth input stage.  Dual path input amplifiers which use a divider before the DC amplifier are even worse (as the AD2 design shows) and make those old dual JFET in a totem-pole configuration used for input buffers look really good.  They even make the Tektronix 7A13 look good.

The Tektronix 7A13 is my reference for oscilloscope vertical amplifiers because it is so noisy.  But it isn't a noisy as a modern oscilloscope!  The 7A13 comes out as better than 20nV/SqrtHz.  Single ended JFET input amplifiers of that age are about 3nV/SqrtHz.  The 7A13 is much worse because it has differential inputs and all of the extra circuitry to support a +/-10 volt common mode input range without input dividers.

What you seem to want is a differential probe which supports using x10 passive probes.  I might have suggestions for an alternative design but it depends on one of the questions I asked:

Is there a specific reason you want to use two channels of the AD2 in a differential configuration?  Why not simplify things and convert the differential input to a single ended output?

This is all very interesting info and it uncovered a whole new world for me as I am not very experienced with instrumentation design. I will try to include a source impedance in the simulator and also to play with the variable capacitor value at the input attenuator of yhe AD2 to see what i can get..

I will agree with you that a single ended output is the way forward, so i look forward to your alternative recommendation

[thanasisk]

Bode response should be flat according to

https://reference.digilentinc.com/reference/instrumentation/analog-discovery-2/reference-manual?redirect=1#figure_14

I will rerun the sim to validate this

[David Hess]

There was another differential probe discussion a few week ago which covers my suggestion below.

My previous experiences using instrumentation amplifiers for differential probing at high bandwidth has never been satisfactory because of common mode rejection and other issues but there is an alternative now other than a discrete design.  Linear Technology and Analog devices make some current feedback based "difference amplifiers" which do exactly what you want with good performance and they are fast but lack high impedance inputs.  Download the datasheets to see how they are used.  I have been considering them so included the Excel file with my notes below.

+/-5 Volt Common Mode Voltage Range:

LT1187   25MHz   Minimum Gain of 2   65nV/SqrtHz
LT1189   18MHz   Minimum Gain of 10   30nV/SqrtHz
LT1193   40MHz   Minimum Gain of 2   50nV/SqrtHz
LT1194   35MHz   Minimum Gain of 10   15nV/SqrtHz
LT6552   37.5MHz   Minimum Gain of 2   55nV/SqrtHz

+/-10 Volt Common Mode Input Voltage Range:

AD830   85MHz   Minimum Gain of 1   27nV/SqrtHz   2V Differential Input
AD8129   185MHz   Minimum Gain of 10   4.5nV/SqrtHz   0.5V Differential Input
AD8130   250MHz   Minimum Gain of 1   12.3nV/SqrtHz   2.5V Differential Input

If you want a differential output with common mode suppression, then I guess you could double up and reverse the inputs just like you did with the LT1102 and this doubles the gain or bandwidth.  I do not see any advantage to driving the AD2 differentially unless you absolutely must have the highest possible dynamic range and even then the improvement by doing so is small.

The AD8129 with a gain of 10 has a bandwidth of almost 200MHz which is not needed but that means it can be configured for higher gains like 50MHz at a gain of 20 and almost 20MHz at a gain of 50 and this has no effect on the common mode input range.  The gain is also easily adjustable with a single ended circuit so the potentiometer trimmer can be grounded on one side.

For high impedance inputs, a preamplifier or buffer will need to be added.  This could be a pair of JFET operational amplifiers configured as buffers or more likely a two operational amplifier JFET differential amplifier with enough gain for a minimum input noise.  It just has to overcome the noise of the difference amplifier.

That leaves one problem which has not been discussed yet when using x10 probes; they don't match producing poor common mode rejection.  Tektronix used to make special probe sets to solve this but a better way in this case would be to trim the 1 megohm input shunts and compensation networks for a dedicated set of probes.  Or you could try selecting a set of probes which match.  X1 probes do not have this issue.

Note that x10 probes effectively raise the input noise of the amplifier by the same amount sort of defeating the purpose if you want low noise at any cost.  The same thing happens with that fixed attenuator on the AD2 inputs.

X10 probes could push the common mode input voltage range out to +/-100 volts.  That is pretty impressive with a potential broadband input noise of 60nV/SqrtHz which is 30 times better than a Tektronix 7A13 under the same conditions although the 7A13 is higher bandwidth.  Note that noise is what limits sensitivity unless DC drift is high.  At low frequencies the 9M resistor in the probe increases noise and there is nothing to be done about that other than not use an attenuating probe.

I could not find any instructions for using the forum "table" feature.

[thanasisk]

To me that looks like it is caused by improper compensation of the input divider.  The corner frequency is about right and the AD2 only includes one compensation adjustment.  High impedance attenuators are greatly affected by the printed circuit board material which is not accounted for in a simulation.  Look up oscilloscope "hook".

Exactly! What I forgot to simulate was the drain and source on/off capacitance of each one of the connections of the ADG612 which add a total of 15pF (typical) of capacitance in parallel with the 5-20pF trimmer. Then I am just 3pF away from a flat bandwidth response and a properly compensated square wave and without attempting to model any parasitics of the pcb. I can confirm that, also in my simulation, even a 1pF change to the compensation cap in the high impedance attenuator is observable both on the frequency response and on the square wave oscilloscope (virtual) instrument.

What do you mean by oscilloscope "hook"? (obviously it is not the hook connectors for the probes..)

[David Hess]

What do you mean by oscilloscope "hook"? (obviously it is not the hook connectors for the probes..)

Getting Rid of Hook: The Hidden PC-Board Capacitance

You probably will not have a problem but it is important where high impedance dividers are used like on the AD2.