Can an analog input range other than those specified in the data sheet be used on the AD719x ?
Yes – the analog input range can be altered. There are two methods to do this:
1) A system calibration can be performed during which the user applies the user-specified zero-scale and full-scale voltages to the ADC during the calibration process. The user must ensure that the fullscale value used is within 0.8VREF/gain and 1.05VREF/gain for specified operation. The ADC will then continue to meet the noise values listed in the datasheet for the original voltage range. For example, the AD7190 has an rms noise spec of 900 nV when operated with an output data rate of 50 Hz (sinc4 filter, chop disabled), an external 2.5 V reference and a gain of 1 which results in a signal range of 0 to 2.5 V in unipolar mode. If the new full-scale value is within 0.8 *2 5V and 1.05 * 2.5V, the rms noise will continue to be 900 nV.
2) The reference voltage can be altered. For example, if an analog input range of 0 to 1.5V is required, using a reference voltage of 1.5V will result in an analog input of 0 giving an output code of 000000Hex and an analog input of 1.5 V giving a code of FFFFFFHex. Following a self-calibration with the 1.5 V reference supplying the ADC, the rms noise will be the same as that for a 2.5 V reference that is given in the datasheet. Using the example above, the AD7190 has an rms noise of 900 nV when operated with a 50 Hz output data rate (sinc4 filter, chop disabled), a 2.5 V reference and a gain of 1. If the reference voltage is changed to 1.5 V and a self-calibration is performed, the rms noise will continue to be 900 nV assuming the output data rate, gain, etc remain unchanged. The peak-to-peak resolution equals 19.5 bits when operated with a 2.5V reference and bipolar mode is selected. With a 1.5 V reference, the peak-to-peak resolution equals log ((2 * 1.5 V) / (6.6 * 900 nV)) / log 2 = 18.9. The reduction in peak-to-peak resolution is due to the reduced input signal span.
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