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