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Part Number: OPA625
Dear Forum Members,I'm writing to you regarding a design problem:We'd like to use the ADS124S08 in one of our products (i.e. sensor signal conversion) and besides converting the analog output signal of a DMS sensor we want to use the internal voltage reference of the ADC as excitation voltage for the mentioned sensor. As the circuit would draw a larger current than the ADC can deliver, I thought about buffering the voltage using one of your operational amplifiers. I want to maintain the high precision of the internal reference, hence I chose the OPA625 because the description stated its use as a "Precision Voltage Reference Buffer".When testing the OPA625, unfortunately I didn't get a satisfactory result. I'm assuming it's a design fault, that's why I need your help.
I use the 3.3V and ground from the ADS124S08 Evaluation Board as supply for the OpAmp, non-inverting unity gain (output -> negative input shortcut) and the ADC internal reference voltage from the EvalBoard as the positive input. The output voltage is not as precise as expected and depending on whether I use a load resistor or not, it varies significantly. What am I doing wrong? Or do you have any other suggestions on how to buffer the reference voltage precisely?Thank you very much and best regards,Manuel
PS: Find the schematic and 3 photos (Measured Internal Voltage Reference, Measured OpAmp Output Voltage with and without load) in the attachment.Attached Photos.zip
The majority of the error you are seeing is likely arising from the 100kOhm resistor you have at the non-inverting input of the OPA625. The OPA625 has a relatively large input bias current (2uA typical). If we assume a typical value then you could expect to see up to 200mV offset at the output because of the bias current.
Zak Kaye Precision Amplifiers Applications
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