What voltage is within specifications on the analog input pins i.e. what is the absolute voltage that can be applied to any analog input pin.
The analog input pins can tolerate a voltage from GND – 30 mV to AVDD + 30 mV when the buffer is disabled and the gain equals 1 or 2.
When the buffer is enabled, the buffer itself will require some headroom. The absolute voltage on the analog input pins must be between GND + 100mV and AVDD – 100mV. The buffer will be non-linear outside this range so the device will be unable to meet the datasheet specifications.
When the in-amp is enabled (gains of 4 or higher), the in-amp itself requires some headroom. The absolute voltage on the analog input pins must be between GND + 300mV and AVDD – 1.1V. If the analog input is outside this range, the ADC will function but the part will not meet the datasheet specifications as the in-amp is non-linear near the power supply rails.
Does this means that I can't measure negative voltages using AD7792?
Basically I'm interested in measuring unbalance in bridge circuit for TCD - http://en.wikipedia.org/wiki/File:Thermal_Conductivity_Detector_1.svg
Any idea how can I use AD7792 for this application? I think signal can be directly connected to the signal of bridge when both have a common ground, the only problem that I can think of is when voltage across R3 or R4 exceeds AVDD.
The AD7792, being a single supply part, cannot tolerate negative voltages with respect to system ground . It can, although, measure negative voltages given that the inputs are biased to a certain dc voltage between GND-30mV and AVDD+30mV in unbuffered mode, GND+100mV and AVDD-100mV in buffered mode, and GND+300mV and AVDD-1.1V when operating with a gain greater than 4.
To measure negative voltages, take note that the signals measured in the AIN(+) input are referenced to the voltage in the AIN(-) input. For example, when the part is configured with a gain of 1 , bipolar mode, and AIN(-) is 2.5V, The signals of AIN(+) can swing from 0V to 5V. If we have a voltage in AIN(+) of 1.5V and having the AIN(-) biased to 2.5V, the input measured will be equal to AIN(+)-AIN(-). With this, we will have a value of 1.5V-2.5V=-1V measured in the inputs.
The AD7792 can be used in this application. You can connect the output of the bridge directly to the input of the ADC, but it is highly recommended that the input signals should not exceed the allowed input range. It is suggested to use the AVDD of the AD7792 as the excitation voltage of the bridge so as to avoid exceeding the input range.
Thanks and Best Regards,
Does it mean when in-amp is enabled it's not able to measure less than 300mv ?!
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