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ADS1263: External Voltage reference question concerning external cap

Part Number: ADS1263
Other Parts Discussed in Thread: REF5025, REF6225

Section 9.3.8.2 applies to an external voltage reference. 

Using this external reference configuration, we are supplying a 2.5 V (approximately) signal between AIN0 and AIN1. The last paragraph in sect 9. 3.8.2 states a 100 nF, or 0.1 uF, should be placed across the input pins.  Is this required? 

We do have and op amp circuit directly connected to the analog pins. If the cap is placed on the output of the op amp, how is the performance impacted?  Data sheet point to Fig 34. 

Although the input is fairly hi Z, adding a isolating resistor between the op amp output and AINx will degrade this voltage.

Please comment on what design practices should be taken in this scenario (if removal of cap is fine, or some other design guidance.

  • Hi Josh,

    I believe the 100 nF differential capacitor is a recommendation, not necessarily a requirement. This capacitor typically provides noise filtering for the reference and can serve as a "charge bucket" to hold the reference voltage stable while it is sampled by the ADC.

    If the reference source is being buffered by an op amp and the op amp is not able to drive this kind of capacitive load (i.e. it may cause the op-amp to oscillate), then it is more important to reduce the capacitance and ensure that the reference voltage remain stable. An unstable reference voltage will lead to unreliable ADC conversion results or very poor noise performance.

    Many buffered voltage references such as the REF5025 will allow for larger capacitive loads (up to 50 uF), and actually require a minimum capacitive load for stability.


    Before recommending that your remove this reference capacitor, I would want to know more about the voltage reference source and if perhaps there is a better configuration that wouldn't require an external op-amp buffer. Unless the current reference source is ratiometric to the input signal, OR unless it is providing a lower noise reference source; I would consider using the ADS12623's internal 2.5V reference or something like the REF5025/REF6225 for lowest noise.