Original question:
Dear TI, I am using OPA2350 for the ADC reference driver of TMS320F28379D. It works very well. However, it is out of stock now. Is there any alternative for it?
I understand that there is special requirement of the OPA used in reference driver, such as high capacitive load capability. Is this true?
HI Jiaxing,
As you correctly have mentioned, the amplifier must be stable driving the reference input bypass capacitor with a small compensation resistor.
The SAR ADC reference input presents a switched dynamic capacitive load to the reference driver circuit. On the SAR ADC architecture, the reference input is sampled several times during each conversion cycle, where the reference voltage is sampled once for every binary weighted ADC bit decision. During each conversion, fast, high-current transients are present in this terminal where the ADC’s internal capacitor array is switched and charged as the ADC bit decisions are made.
For this reason, the reference driver circuit must keep the reference voltage settled without significant droop (less than 1-LSB) between each of the SAR ADC clock cycles, and without excessive droop between each complete ADC conversion cycles (a few LSBs of droop after many complete conversion cycles is acceptable in most applications). The compensation resistor has to be large enough to keep the amplifier stable, but also needs to be kept relatively small (typically less than 1-Ohm) to keep the reference settled. This compensation resistor is typically in the several 100s of milliohms range. There is a more detailed explanation on the post below:
The OPAx350, OPAx320,OPAx325 and the new OPA2328 are stable driving the 22μF reference bypass capacitor with a small compensation resistor. Use a 22μF reference input capacitor at the (VREFHI) input of the TMS320F28379D and a small series resistor in close proximity with the REFHI input pin of the microcontroller.
When using the OPAx320 use 22μF and a small series resistor of >360milliOhm right at the reference input of the microcontroller.
When using the OPAx325 use 22μF and a small series resistor of >500milliOhm right at the reference input of the microcontroller.
Please also consider the OPA2328. This is a new 40-MHz precision amplifier that can easily drive the 22μF bypass capacitor with a small compensation resistor. See attached a TINA-TI simulation with the OPAx328 stability analysis driving the 360milliOhm+22μF bypass capacitor.
Attached is a pdf file with the analysis for the OPAx320 and OPAx325 stability and reference drive analysis.
Thank you and Regards,
Luis
