Author Topic: Preamp for Analog Discovery 2  (Read 11776 times)

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Offline thanasiskTopic starter

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Preamp for Analog Discovery 2
« on: July 18, 2018, 12:05:50 am »
I am investigating the use/design of a preamp for measuring low-level (<10mV) low-frequency (<10 MHz) signals with the Analog Discovery 2. The idea is to boost the measured signals above the noise floor of the unit.
The AD2 has an Input impedance of 1MOhm || 24pf but unfortunately the sensitivity is +-2.5V full-scale (500mV/div) in “high gain mode”.

Theoretically a 1000x(60dB) gain could yield 500uV/div.
Or a 250x gain for 2mV/div.

In general I found numerous preamp designs based on op-amps and jfets+op amps and paralleled op-amps and other mic preamp evaluation boards (from thatcorp). They offer excellent noise+THD performance BUT they all have been specifically designed for low source impedance (1KHz..10KHz).

In which case, if  a 10x probe is used at the input or if the test point is high impedance, then the accuracy of measurement due to loading and the noise & distortion of the preamps would severely suffer.

The Renesas ISL28634EV2Z also looks nice (differential-in differential-out) but the gain-bandwidth of the instrumentation amplifier used is not that great (1000x at up to 1 KHz approx.)

- Any thoughts/ideas/recommendations?
- If I have to design something from scratch, do you think a single low-noise op-amp stage (or instrumentation amp) with 60dB gain at >20KHz be feasible?
- Also am I right to look for op-amps which perform well with high source impedance of approx. 10MOhm to support a 10x probe and/or high impedances for probing with a 1x probe? See Fig. 2 of  http://www.analog.com/media/en/reference-design-documentation/design-notes/dn355f.pdf
- Or alternatively would you recommend to look at discrete jfet designs? Such as Fig. 4 from http://www.janascard.cz/PDF/Design%20of%20ultra%20low%20noise%20amplifiers.pdf

« Last Edit: July 19, 2018, 10:02:56 am by thanasisk »
 

Offline rstofer

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Re: Preamp for Analog Discovery 2
« Reply #1 on: July 18, 2018, 12:14:46 am »
Dave has a video on extending bandwidth by limiting the gain of each stage.  So, more than one stage...



Dave also has the uCurrent gadget.  You can search for it but here is the schematic

http://www.eevblog.com/files/uCurrentRev5schematic.pdf

 
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Offline JS

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Re: Preamp for Analog Discovery 2
« Reply #2 on: July 18, 2018, 12:23:54 am »
AD620 might be a good option, ~90k source 20dB seems to be the sweet spot for noise (not for offset) and then you can go for a few extra stages with opamps, let them clip, maybe with some diodes to just select between different stages for the sensitivity señection. If you use higher rails you could use the attenuator in the analog discovery to select between intermediate steps.

JS

 If using an instrumentation amp you could take the go to make it differential
If I don't know how it works, I prefer not to turn it on.
 
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Offline EmmanuelFaure

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Re: Preamp for Analog Discovery 2
« Reply #3 on: July 18, 2018, 02:39:36 am »
Some ideas :
AD8428 : Bandwidth 3.5 MHz at fixed gain = 2000. But not adapted to high source impedance (Bipolar input with high bias current and current noise).
LT1102 : Bandwidth 3.5 MHz at fixed gain = 10. JFET input, so well adapted to high source impedance.
 
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Offline cat87

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Re: Preamp for Analog Discovery 2
« Reply #4 on: July 18, 2018, 06:29:19 am »
How about this? 

http://techlib.com/electronics/audioamps.html

Also,  if you want to go the in-amp route,  maybe the INA111 or INA112?
 
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Offline DaJMasta

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Re: Preamp for Analog Discovery 2
« Reply #5 on: July 18, 2018, 07:41:47 am »
If you find something you like that needs a low source impedance, you can always put a high impedance buffer as a frontend.  If your buffer stage is either unity gain or low gain, it doesn't need nearly the gain bandwidth product as your main amp, so it should be easier to find something that's low noise and has very little current requirements from your probe.  I definitely think cascading amps is going to be your best bet if you're needing several MHz of bandwidth and very low noise - most of the amps with several hundred MHz GBP required to get good gain and frequency response in one stage generally have higher noise figures near DC (audio amps are often spec'd at 1kHz or 10kHz whereas faster amps are spec'd at 1MHz and the noise figure generally goes up exponentially towards DC), so while they'd be fine as a high gain stage for a reasonably fast AC signal, you're going to lose the low level fidelity near DC.  Using amps designed more for low frequency use will give you much better noise performance (and when looking at very small signals, this will be important), but will generally have much lower gain bandwidth product, giving you much lower maximum gain for a given bandwidth requirement.


It's also probably worth including a low pass filter for your intended maximum bandwidth in your frontend - reducing the overall bandwidth of the system inherently reduces the noise floor, so limiting it to the frequency span you want to look at will improve low noise performance.  Looking at the very small signals also means you're much more susceptible to grounding issues, supply noise, and EMF.... so be careful the way you layout the design.  Local regulation, ample decoupling, an analog ground plane with a wide single path to ground, and maybe even shielding could be beneficial to the overall performance of the design.
 
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Offline tggzzz

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Re: Preamp for Analog Discovery 2
« Reply #6 on: July 18, 2018, 11:09:01 am »
Don't forget to calculate the unavoidable noise (shot, thermal) in your front-end.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
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Offline David Hess

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Re: Preamp for Analog Discovery 2
« Reply #7 on: July 18, 2018, 02:40:44 pm »
- Any thoughts/ideas/recommendations?

Check out the Tektronix AM502 which can provide a gain of 100,000 (100db) with a 1 MHz bandwidth.  It was made for exactly this type of application.

Quote
- If I have to design something from scratch, do you think a single low-noise op-amp stage (or instrumentation amp) with 60dB gain at >20KHz be feasible?

Not without a lot of care in design and construction.

Quote
- Also am I right to look for op-amps which perform well with high source impedance of approx. 10MOhm to support a 10x probe and/or high impedances for probing with a 1x probe?

There are some suitable JFET input operational amplifiers but watch out for variation of input bias current and input capacitance with signal level.  Low frequency noise will be a limitation and MOSFETs are worse in this regard.  High open loop bandwidth in the first stage to get high gain will be counterproductive because high bandwidth devices are also higher noise.

Quote
- Or alternatively would you recommend to look at discrete jfet designs?

Check out the Tektronix AM502 for an example of a discrete JFET design.  Notice that neutralization was used and gain of the JFET stage is relatively low.

A discrete JFET input design could have 1/4 the noise of an equivalent integrated JFET design but other considerations are more important.

Quote
Such as Fig. 4 from http://www.janascard.cz/PDF/Design%20of%20ultra%20low%20noise%20amplifiers.pdf

That is AC only.

I suspect you are going to want a differential input like the AM502 to avoid noise contributed by ground loops.
« Last Edit: July 18, 2018, 02:45:23 pm by David Hess »
 
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Offline JS

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Re: Preamp for Analog Discovery 2
« Reply #8 on: July 18, 2018, 11:54:35 pm »
...
The AD2 has an Input impedance of 1MOhm || 24pf but unfortunately the sensitivity is +-2.5V full-scale (500mV/div) in “high gain mode”.

Theoretically a 1000x(60dB) gain could yield 5uV/div.
Or a 250x gain for 2mV/div.
...

You forgot a few zeros somewhere, 500mV/1000 is 500µV, not 5µV. How low is the noise from the AD2? I guess you would want to match that noise or there about, there's no reason to go much under as you will get the same noise, and there's no reason to get much higher or you are amplifying more than it's useful.

Still, not very low noise. If you are ok with 1MHz bandwidth it's not that hard, many options for low noise, 10MHz opamps, using a gain up to 10x on each stage and you will be fine. Using the OP27 as first stage will give noise floor for 8 bit resolution at 60dB final gain @ 1MHz BW and be good down at DC. If you implement a selectable filter you can get even better. Going up to 10MHz is a bit harder as low noise opamps are harder to find and more expensive, and close to DC aren't as good anymore.

JS
If I don't know how it works, I prefer not to turn it on.
 
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Offline cat87

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Re: Preamp for Analog Discovery 2
« Reply #9 on: July 19, 2018, 06:32:22 am »
Just throwing this in the pile, maybe it'll be useful to you at some point.

If you want to go all the way down to DC and up to say 10 or 20 Hz,  you can use a split path amp. You can have you ac-coupled amp,  say 10Hz to 1MHz  then,  also have something like a LTC1050  handling the DC part,  in parallel. Identical gain,  of course.

There's something  regarding this in App Note 106 fron  Linear.
 
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Offline mtdoc

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Re: Preamp for Analog Discovery 2
« Reply #10 on: July 19, 2018, 06:47:07 am »
Check out the Tektronix AM502 which can provide a gain of 100,000 (100db) with a 1 MHz bandwidth.  It was made for exactly this type of application.

+1 for the AM502. The selectable low and high frequency filters are a boon as well.
 

Offline thanasiskTopic starter

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Re: Preamp for Analog Discovery 2
« Reply #11 on: July 19, 2018, 09:24:56 am »
Thank you all for the excellent pointers!

Yes of course I meant 500uV/div  ;D

The AM502 looks nice but needs a TM501 power module if I am not mistaken to go with it. I have seen listimgs on ebay but, because of the age of the devices, it is completely unclear whether any sold device performs according to spec.

I am not sure about the noise floor of the AD2 since I do not currently have the device and all measurements by other users online were not terminated properly. Of course this is essential info needed to set the design goals for this project. Any user of AD2 out there care to measure the AD2 noise floor?

 I realize that a nicely optimized two stage cascade would be the best option. A high impedance low noise  1x or 10x stage followed by a higher gain stage.

Since the inputs of the AD2 are differential, a cascade of diff in diff out instrumentation amps would be ideal. And a switchable bandpass or low pass filter would further limit the noise.

That is a lot of stuff to study and there are a lot of options to consider.. 

By the way, I also saw this evaluation board which offers 60db gain at sufficient BW

http://www.analog.com/en/design-center/evaluation-hardware-and-software/evaluation-boards-kits/eval-ad8253.html#eb-documentation

But I cannot find it in Europe..




 

Offline xani

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Re: Preamp for Analog Discovery 2
« Reply #12 on: July 19, 2018, 03:34:34 pm »

I am not sure about the noise floor of the AD2 since I do not currently have the device and all measurements by other users online were not terminated properly. Of course this is essential info needed to set the design goals for this project. Any user of AD2 out there care to measure the AD2 noise floor?


If you tell me what exactly do you need, I can do it, I'm not an expert on low noise measurement so I assume you need a bit more than just to short output and measure that ?

I was toying with similar idea as yours, I wanted to make a "front panel" with a bunch of switches (basically nicer version of AD2's BNC adapter) and possibly a front end for it.
 

Offline JS

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Re: Preamp for Analog Discovery 2
« Reply #13 on: July 19, 2018, 06:02:10 pm »
I don't kbow the optimal source impedance of the ad2, but having a noise measurement with shorted input and maybe 2 or 3 with other impedances would do. 1k, 10k, 1M would be my choices in the dark, from those thw optimal could probably be extracted. Just a resistor in the input, lower range, rms and peak to peak will give more than enough information. The knee for 1/f would be sweet but I don't think it's essential.

JS

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Offline xani

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Re: Preamp for Analog Discovery 2
« Reply #14 on: July 19, 2018, 07:18:59 pm »
From schematics it seems to just have ~1M input with a resistor divider followed by analog switch and FET amp so it is likely that input impedance won't matter much.

I'll try to get it measured this evening, provided today's day at work wont have more suprises  |O
 

Offline David Hess

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Re: Preamp for Analog Discovery 2
« Reply #15 on: July 19, 2018, 09:02:13 pm »
There is no reason to match the impedance when a high input impedance buffer is used; by the time you have a low enough impedance to drive the input capacitance at the bandwidth you require, noise considerations are insignificant and the only thing which matters is the input voltage noise which can be calculated from the AD2 schematics.  Broadband noise starts at about 33nV/SqrtHz because of the minimum input divide by 5 stage.
 

Offline JS

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Re: Preamp for Analog Discovery 2
« Reply #16 on: July 19, 2018, 10:52:05 pm »
I was treating it as a black box, haven't look the schem...

33nV/sqrt(hz) then the AD620 has 13nV/sqrt(hz) so you only get a gain of 3 not good.

JS

JS

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Offline thanasiskTopic starter

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Re: Preamp for Analog Discovery 2
« Reply #17 on: July 19, 2018, 11:14:08 pm »
My recommendation for the noise measurement would be to use the BNC adapter board (and not the flywires) and to terminate the input with any BNC terminator you have available. Any terminator: short circuit, 50Ohm, 75Ohm or whatever you may have available will not affect the noise measurement at all because the equivalent source resistance for the input buffer will not be severely altered.

The AD8066 input buffer used has a corner frequency of around 2kHz (see fig. 21 in the datasheet). I believe this is the noisiest component in the chain.

 

Offline xani

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Re: Preamp for Analog Discovery 2
« Reply #18 on: July 20, 2018, 12:45:48 am »
Okay, so termination are just 2 jumpers directly on AD2 socket. Last one is 1MHz sine fed from SDG1025 via BNC adapter



 
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Offline JS

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Re: Preamp for Analog Discovery 2
« Reply #19 on: July 20, 2018, 12:55:22 am »
Pictures in tapatalk sucks, really!
I'll need to come back in a few days on my pc, but I will put some effort into this as I do want to build a preamp of similar specs for my setup...

JS

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Offline David Hess

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Re: Preamp for Analog Discovery 2
« Reply #20 on: July 20, 2018, 01:41:07 am »
The AD8066 input buffer used has a corner frequency of around 2kHz (see fig. 21 in the datasheet). I believe this is the noisiest component in the chain.

The breakpoint for thermal noise in the input divider and series protection is higher than the 1/f corner frequency of the operational amplifier.  Assuming I did it right, below 320kHz noise rises to 12nV/SqrtHz and below 7.3kHz, noise rises to 55nV/SqrtHz so the 1/f corner frequency is actually about 30Hz.  Most oscilloscope inputs avoid this level of noise by not using an input divider on their most sensitive ranges and including a lot more bypassing on the series protection resistor although this makes input protection more difficult.

As a practical matter, this means excluding quantization noise, the external preamplifier should have enough gain to overwhelm 55nV/SqrtHz if performance below about 7.3kHz matters and that is the design constraint I would start with.  For an integrated JFET amplifier, that means at least a gain of about 10 minimum.  For a lower noise discrete JFET, that means a gain of about 50 minimum.

Update: Those were the noise values at the input to the operational amplifier.  The input referred noise for the AD2 is 5 times higher because of the attenuator so 60nV/SqrtHz below 320kHz and 275nV/SqrtHz below 7.3kHz.  So for an integrated JFET amplifier, that means at least a gain of about 50 minimum and for a lower noise discrete JFET, that means a gain of about 250 minimum.
« Last Edit: July 20, 2018, 11:45:45 am by David Hess »
 
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Offline Krytron

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Re: Preamp for Analog Discovery 2
« Reply #21 on: July 20, 2018, 07:04:02 am »
Look at the JFET designs:  internet search "Design of Ultra Low Noise Amplifier, Vojtech Janasek, www.janascard.cz" under downloads (on the main page to the right is button for english).
« Last Edit: July 20, 2018, 07:07:08 am by Krytron »
 

Offline thanasiskTopic starter

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Re: Preamp for Analog Discovery 2
« Reply #22 on: July 20, 2018, 02:58:16 pm »
Thank you all!
I ll try to come up over the weekend with a rough first design with differential inputs and outputs using instrumentation amps using all this info here..
Later i can look at more elaborate discrete designs..

I am wondering whether the IN+, IN- and GND inputs should be BNC connectors for connecting typical oscilloscope probes..  Does it make sense? I saw in a design by Walt Jung (In regulators for high-performance audio, 1995) that he used shielded twisted pair for direct probing.
« Last Edit: July 20, 2018, 03:03:52 pm by thanasisk »
 

Offline xani

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Re: Preamp for Analog Discovery 2
« Reply #23 on: July 20, 2018, 03:15:53 pm »
Well if you wanna go all in you could possibly make an active probe-type of solution with head that has say x100 amplifier sending that to the rest via twisted pair or BNC.

But I'd say test with just a normal BNC + scope probes and only bother upgrading from that if that is not enough. Also there is always option of just soldering BNC cable directly to the target
 

Offline thanasiskTopic starter

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Re: Preamp for Analog Discovery 2
« Reply #24 on: July 22, 2018, 12:47:32 am »
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/)


Thanks David for the very detailed pointers. They were very useful to understand what is going on with the noise calculations.


Now lets see how a single stage differential-single ended preamp based on the LT1102 (input referred noise voltage of 20nV/sqrt(Hz)) could perform:

- One LT1102 configured as G=10 (3.5MHz BW) would offer a 50mV/div sensitivity to the AD2. Output referred voltage noise of 200nV/sqrt(Hz) would mean approx 374uVrms@3.5MHz BW. However, performance below 7.3kHz would suffer because the gain is not enough to push over the AD2 noise floor in that region.

- One LT1102 configured as G=100 (220kHz BW) would offer a 5mV/div sensitivity to the AD2. Output referred voltage noise of 2000nV/sqrt(Hz) would mean approx 938uVrms@220kHz, offering 10.6 bits ENOB.


A differential out configuration similar to p. 9 of the datasheet would double the gains to 20/200 but suffer from increased voltage noise.

- Two LT1102 configured as G=20 (3.5MHz BW) would offer a 25mV/div sensitivity to the AD2. Output referred voltage noise is 566 nV/sqrt(Hz).

- Two LT1102 configured as G=200 (220kHz BW) would offer a 2.5mV/div sensitivity to the AD2. Output referred voltage noise is 5657nV/sqrt(Hz).

Edit: updated the noise calculations for the differential output case
« Last Edit: July 23, 2018, 02:26:42 pm by thanasisk »
 


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