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Question asked by python on Oct 15, 2012
Latest reply on Oct 16, 2012 by ChrisD.Rama

Hi,  i have two questions about AD7798

1. we know that AD7798 doesn’t contain a Vbias at the input front-end. if the customer wants to detect a signal-ended small signal(mV level,need 128 gain stage), he needs to add a Vbias to the input to satisfy the input requirement(can be seen in the attached). Figure in the attached comes from the datasheet of AD7793(not ad7798), the input has a Vbias voltage and it add to the AIN-, I don’t know why this voltage only add to one input? I mean it should be added to both two input pins(AIN+ and AIN-), and can be canceled as a common voltage by the following IN-AMP. if the customer uses AD7798, does he need to add a bias like the AD7793's internal block? or need to add a single-to-differential OP? Pls give me some suggestions about this.

2. What’s function of the two current sources between the INA-AMP's input and the power(both power supply and GND) that i have marked in the attached, i can see them in most precision sigma-delta ADC's  functional bloack diagram and I can’t understand why that exist.