I'm developing a board which is based on ADS1281 ADC. The board is for research purpose in the geophysical field. I have looked at “ADS1281EVM and ADS1281EVM-PDK User's Guide” as reference project. My question is: why the mentioned project uses an active filter for the reference voltage?
I have already read the document “Filter your voltage reference for low-noise performance”, by Ron Mancini, which discusses the topology of filter which is used in the reference project. The documents tells that active filters are used, rather than passive filters, in order to improve output control.... To be exact, Ron Mancini says that active filers work better than passive ones at very low frequencies. In other words, the operational amplifier in the filter removes the errors due to the voltage drop across the passive filter. This voltage drop is a DC signal (average current sync by reference input of the ADC) superimposed with a low frequency fluctuation ( thermal effects on the output capacitor ).
The reference project features a passive filter cascaded with an active filter. Note that the passive filter has lower cut-off frequency than the active one. Maybe I am wrong, but, in my opinion, the reference project would perform the same if the active filter were replaced with a passive filter with R=47 and C=100n...
I suppose that something is wrong in my reasoning but I don't know what is it....
The filter shown on the ADS1281EVM is implemented with a simple RC filter, followed with a buffer op-amp with a RC filter at the op-amp output; somewhat similar to the one described on the application note mentioned. The first passive RC filter provides a lower cut off frequency pole. The buffer has a RC filter at the output with a higher frequency pole as you mentioned, but in this case, most of the filtering is performed by the first stage lower corner frequency RC filter.
The OPA227 has a small input bias current in the order of a few nanoamps, therefore, adding the buffer reduces any voltage dropout errors that would occur on the filter's 10kOhm resistor. The Reference inputs of the ADS1281 has an effective input impedance in the order of 85kOhm, therefore the reference inputs may draw currents in the microAmp range. The ADS1281 reference input impedance is proportional to the clock rate. This current can cause a voltage drop accross the filter's resistor and cause some regulation/drift errors. Adding the buffer in this case, allows for placing a larger filter resistor, since the input current of the buffer is in the order of a few nanoamps...
Attached are a few slides regarding other Reference circuits you could use for the ADS1281/2 devices using the REF5050 and REF02 with passive low-pass filters. Depending on the power budget available and the error budget in your application, you could choose to use a passive filter or add an external buffer similar to the one shown on the ADS1281EVM.
First of all, tank you for your quick response. The slides have been very helpful, especially the temperature drift graph. I'd like to put another question to you. The project I'm working on will have four input channels. In order to save power the reference circuit should be shared among the channels. Consider that I'm going to use the filter architecture shown on ADS1281EVM. In your opinion, can a single active filter drive four channels without loosing precision?
If the four ADS1281 devices will share the same voltage reference, you will need to place a 0.1uF capacitor directly across each device reference inputs (VREFP and VREFN).
Since the Operational amplifier is buffering the current to drive the reference inputs and the filter resistors are inside the amplifier's feedback or before the buffer, there should not be significant error due to the filter's resistors. Provided that the devices reference inputs are in close proximity to the Voltage Reference circuit , this should work well.
you have been very kind, thank you...
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