AD7665
Production
The AD7665 is a 16-bit, 570 kSPS, charge redistribution SAR, Analog-to-Digital Converter that operates from a single 5 V power supply. It contains a high...
Datasheet
AD7665 on Analog.com
AD8031
Production
The AD8031 (single) is a single supply voltage feedback amplifier that features high speed performance with 80 MHz of small signal bandwidth, 30 V/µs slew...
Datasheet
AD8031 on Analog.com
The datasheet has in figure 5 the very strange decision to hang large 10 uF and 22 nF off the output of the OpAmp despite the fact that this is order of magnitude outside the OpAmp's spec, and normally doing this would cause an Amp in a G=1 Buffer config to oscillate.
Previously, a customer asked about this:
Recommendations from datasheets for AD7665 and AD8031 contradict each other
However this answer doesn't really help. Yes, the assertion is made that the AD8031 will be stable, but no reasoning or qualifications are given.
1) Does the type of capacitor matter? High ESR, low ESR, etc..
2) Why do these large capacitors need to be here? Is it *because* the AD8031 has a large output impedance?
Understanding what is going on here is needed to select appropriate components
Hi, adamvs .
For a better reference, you can try checking its design resources from the product page of its evaluation board.
EVAL-AD7665 Evaluation Board | Analog Devices
A reference buffer can be used when the drive of the chosen reference is insufficient. And for this, you need to compare both output impedances of the chosen reference and the maximum allowed output impedance for a ½ LSB voltage drop of the ADC:
ADR43X: (5V x 15 ppm/mA) / 1000 = 0.075 ohms, where 5V is the Vin and 15 ppm/mA is the load regulation
AD7665: (2.5/2^n+1)/114 uA = 0.167 ohms, where 2.5/2^n+1 is the 1/2 LSB voltage.
--> 0.075 > 0.167
Comparing both of them, ADR43X has enough driving capability since its output impedance is lesser compared to AD7655. On the other hand, the purpose of AD8031 to the reference pin is to ensure there is enough stability to the pin during charge redistribution and switching during the sampling of the ADC, which causes current to be drawn by the reference. Capacitors at the output of the op-amp serves as a decoupling capacitor the Ref pin, just like the typical configuration of power supplies. Also, these capacitors keep the reference voltage free of noise. Configurations to this is placing high value and low ESR reservoir capacitor, with a typical value of 10 uF placed close as possible to the reference input.
Regards,
Jo
Sorry, I don't follow your answer at all.
I asked about whether there are any requirements for the capacitors at the output of AD8031. They are probably NOT low ESR caps as this would create a much higher likelihood of oscillations. The eval board image is blurry but I think C8 it is a tantalum cap. which would make more sense.
----------regarding your response-----------
The datasheet says the purpose of the buffer is
It does not (strictly) have anything to do with the maximum current which the reference can provide. It is there to isolate the reference output from possibly rapid changes current draw from the in put signal, which flows from one or more of IN(A,B,C), through IND depending on the voltage range used.
In the absence of this buffer the finite load regulation of the reference will cause the REF pin to vary with the applied signal.
