ADS1282 EVM reference

Hello Reto, 

the RC low pass filter connected after REF5050 is able to reduce the noise from the REF5050. This improvements brings the noise level under control. However, the current from REF goes through the R3 and it cause voltage drop which is equivalent to average charge level from the reference pin of the ADC.  Therefore, the circuit will not be able to drive ADC voltage-reference input adequately. For this reason the output buffer will need to be added to low-pass filter. 

Reference will be powerful enough for driving the ADC.

If I am right, the operational amplifier which is used as buffer is type OP227. Based on the noise performance characteristics., its noise performance is 90 nVp-p or input noise spectral density equal 3-3.5 nV/√Hz. To my mind, this is his noise contribution to the overall noise level.  The noise performance of the REF5050 is 15 micro Volt p-p

I you are interested in the further study of this topic, you should go through these documents

http://www.ti.com/lit/an/slyt331/slyt331.pdf

http://www.ti.com/lit/an/slyt339/slyt339.pdf

http://www.ti.com/lit/an/slyt355/slyt355.pdf

Best regards, 

Roman 

Roman,

Thank you very much for the response and good explanation, and sharing the articles.

Hi Reto,

The Reference circuit used in the ADS1282EVM consists of a buffer with two RC filters. The buffer has a low pass filter at the input of the buffer and a second low pass filter at the output of the buffer.   In this circuit, 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, as explained in the post above, adding the buffer reduces any voltage dropout errors that would occur on the filter's 10kOhm resistor.  The Reference inputs of the ADS1282 has an effective input impedance in the order of 85kOhm, therefore the reference inputs may draw currents in the microAmp range.  The ADS1282 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.

Please see a detailed presentation attached regarding the voltage reference of the ADS1281/2,

Thank you and Best Regards,

Luis
2703.1385.ADS1281-2_volt_ref_forum.pdf