If using the ADR421 voltage reference with the AD7190, what is the recommended value or range of values for a decoupling capacitor? Also, how is the input impedance of the reference inputs derived?
The reference input is sampled at the modulator frequency which is 307.2 kHz on the AD7190. Therefore, the current drawn on the reference pin is not stable. The front-end switched in the reference every 307.2 kHz (modulator sampling frequency) and samples the reference voltage. When the reference is first switched in, there will be a larger current which will reduce with time. The datasheet lists the average value.
This sampling frequency, the normal mode rejection of the reference is the same as the analog inputs.
The datasheet lists the acceptable RC values for the analog input in unbuffered mode. The reference inputs can tolerate similar values.
Please let me know if there is additional information that you require.
Regarding with the 4.7 uF cap, it is suitable on the reference output when it is connected to the AD7190. This capacitance is required by the ADC. The ADR421 itself can accept a wider range of capacitances (consult the ADR421 datasheet for more details).
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A 4.7 uF capacitor is used on the ADR421 output on the AD7190 evaluation board. We have measured the noise performance with this reference and it meets the datasheet spec. So, I suggest you use a similar value if you are using the ADR421 with the AD7190.
The input impedance is derived by measuring the leakage current on the reference pins. The leakage current is 7 uA/V typical. This leads to an impedance of 1/7uA = 142Kohms.
Thank you very much for the response. In some applications, I won't be using an ADR431 type reference, so for the input impedance, I am interested in the frequency dependent nature. The data sheet mentions that the input impedance is dynamic and that certain R-C combinations on the input can cause DC errors. I would like to better understand this so that I can derive acceptable source impedances.
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