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4 channel ADC, 10 MHz, 8 bit design
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FrankBuss:
I'm planning to build a 4 channel ADC shield, for the Microsemi Creative FPGA board, which has Arduino compatible headers (with 3.3 V for the digital IOs). I would like to sample +/-10 V with 10 MHz samplerate. No need for gain etc., I plan to use it to record the programming sequence of a microcontroller programmer. With the integrated RAM on the Creative board I can sample 64 MB, so more than a second for 8 bit resolution and 10 MHz samplerate, and then I can transfer it over UART to a PC. I don't need more at the moment.

Attached is a first draft of the circuit diagram, with one input stage and one ADC. It is mainly copied from the application notes from the datasheets, except for the ADA4622, which I'm planning to use for the input stage. Is this sufficient for a few MHz analog bandwidth and does the rest look good? I'm not sure about the input range and if U2 does the right voltage shifting, I have no idea how a current feedback OpAmp works. Calibration etc. is no problem, I will do this in software.
Kleinstein:
It  don't think U2 will do much level shifting. It's essentially a differential amplifier with a little gain (e.g. 1.5 times).  R12 and R18 should be likely swapped. I would expect some limiting here - chance are U2 would also give out negative voltages. Level shifting would need some kind of DC reference level (possibly derived from the reference).

There is no real need for R9, unless there is a longer coax line between U4A and R9.

The LMC662 for the reference buffer is an odd choice. Q2 is also the wrong way around.

The input divider with R1,R3 would need an additional adjustable (trimmer) capacitive divider in parallel to get some compensation.

For better channel separation and less concentrated heat I would not use the ADA4622-4 but more like 4 separate chips.
FrankBuss:
Thanks for the review. The LMC662 is from the datasheet example of the ADC: https://www.ti.com/lit/ds/symlink/adc1175.pdf see page 18. Why is it an odd choice?

The LMH6702 is also in the datasheet as an example on page 16. But I guess they assume only positive input then? How do I need to change it to allow positive and negative input?

Do you have an example for the additional capacitive divider for R1, R3?

Good idea with the ADA4622, I will change it to 4 separate ADA4622-1 ICs.
David Hess:
Your VRB generator circuit is well thought out but the PNP transistor is reversed.  The reference circuits may need a higher minimum load for stability than the reference inputs of the ADCs will provide.

U2 is configured as a non-inverting amplifier and not level shifting anything so that needs to be changed.  Usually the singled ended reference to the ADC is divided by 2 and fed to the level shifter to move the signal up by half of the ADC input range but that will not work here.  It may be easier to use an ADC which has a bipolar input range simplifying the reference but a level shifter can be made to work.

The high input impedance divider is going to require frequency compensation to maintain the same attenuation at higher frequencies.  A small fixed capacitor across R1 and a trimmer capacitor R3 (or the reverse) will work; calibrate with a square wave test signal.


--- Quote from: FrankBuss on November 24, 2018, 04:37:42 pm ---The LMC662 is from the datasheet example of the ADC: https://www.ti.com/lit/ds/symlink/adc1175.pdf see page 18. Why is it an odd choice?
--- End quote ---

The extra low input bias current of the LMC662 is not required and it is not a low noise precision part.  But since the ADC resolution is only 8 bits, the LMC662 should be fine.  The LMC6082 is a higher precision LMC662 and would be better.
FrankBuss:
Attached is a new version of the circuit diagram, and a Spice simulation. I added the compensation capacitors, are the values right? And I fixed the wrong transistor orientation, added some load resistors to VRT and VRB, just in case they are needed, and added an offset to the ADC driver OpAmp. First I tried to generate the offset with the LMC662, but it was not strong enough, so I use the LMH6702 for this as well. I found Spice models for it and could simulate it with LTSpice, at least in the simulation it looks good :) but it clips at about 3.5 V, I hope this is sufficient. But with the 2 new pots I can even adjust the gain and offset perfectly, so I don't need to do this in software.

But now I wonder if the ADA4622 can drive the low impedance input of LMH6702? There is a Spice model as well for this OpAmp, but don't know if it is worth the effort to simulate this, if someone can tell me if it works.
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