Can I use instrumentation amplifier AD8xxx to drive the ADC AD7xxx, or do I need to use an ADC driver stage in between them? If the instrumentation amplifier can drive the ADC, what should the values be for the RC filter between them?
This question mainly depends on the AC performance you want out out your system. Specifically:
- What kind of distortion performance do you need
- How fast your system needs to settle after a transient step.
Most applications using sigma delta ADC's are pretty slow, and these applications rarely demand much AC performance. Sigma delta converters also often have a on-board buffer amplifier specifically designed to drive the ADC. This question mainly arises with our SAR (successive approximation converters) which have pretty fast sampling ranges (100's of kHz and beyond), and do not have on board buffers.
Converters typically like an external capacitor connected to their input to supply instantaneous charge to the ADC's sampling capacitor. But amplifiers do not like driving capacitive load. The solution is to use a resistor in between the capacitor and amplifier to prevent the amplifier from going unstable. This is why most SAR ADC datasheets recommend an RC filter network between the driving amplifier and ADC.
A rough rule of thumb is to use an RC filter with a bandwidth 5x lower than the bandwidth of the instrumentation amplifier in its lowest gain. (This is just a starting point, and should be optimized on the bench for the specific in amp - ADC combination.) Most instrumentation amplifiers are designed for precision and low power - not high speeds, which means that the RC filter may need to be a fairly low frequency. This has two negative implications for AC performance: 1) The resistor value in the filter is probably higher than the SAR ADC datasheet recommends, which will cause the ADC to distort. 2) This filter may slow down your settling time.
While it is possible to do hand calculations to get a good estimate of the settling time, it normally requires bench testing of the specific amplifier, RC filter, and ADC to determine the circuit's distortion performance.
Occasionally it may be possible to eliminate the RC filter altogether and drive an unbuffered ADC directly. This depends on both the instrumentation amplifier's drive ability and the structure of the ADC's input front end, and requires testing on the bench to determine. For any unbuffered ADC, It is good practice to always populate your PCB for the RC filter option.
Driving a differential ADC
To get extra dynamic range, many higher performance ADC's now have differential inputs. We have three instrumentation amplifiers fully specified for differential operation: AD8295, AD8222, and AD8224. In addition, by adding two resistors and an op amp, most of our instrumentation amplifiers can be converted to a high precision differential output instrumentation amplifier. For examples of how this is done, see the AD8221 or AD8226 datasheets. If using an op amp with lower bandwidth than the in amp, then the op amp will likely set the RC filter cutoff.
As an alternative to using a standard instrumentation amplifier, another option is to use a differential driver optimized for ADC driving like the AD8475. The AD8475 inputs can be buffered with op amps to get a high input impedance circuit.
There is one caveat with capacitor choices when it comes to the actual implementation. In general, for best performance on the signal path, COG/NP0 capacitors are preferred. Not only they would have the lowest temperature coefficient, but have lower voltage coefficient and less dielectric absorption than other ceramic capacitors. These characteristics translate into higher precision for DC applications and lower distortion for AC applications. The drawback is that they tend to be the biggest in size compared to other ceramics. Additionally, the bigger these capacitors, the higher the tendency they have to show microphonic effects (they pick up vibrations and translate them into electrical signals). If board area is a constraint, choosing large capacitance values may become an issue as well.
It is always best to find a good compromise between resistor size and capacitor size when choosing values for the RC filter, as they would be critical to the overall performance of the circuit.