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Biasing ADC input

T

thomas

Jan 1, 1970
0
I have a AD594 temp sensor (~10mV/C output) connected to the 10bit ADC on an
AVR. The sensor output will be -375mV to +1015mv for the temperature range
(-40C to +100C) I'm interested in. The ADC only accepts +V analog input so
I'll need to bias the signal 'up' a little to get it above 0V. If possible
the solution should be repeatble in production and not require individual
calibration.

Specs:
Using a TC7660 or TCM829 for the negative supply to the AD594.
I'm using the internal 2.56V Vref of the AVR ADC. Vref is exposed on a pin.
The AD594 can source 5ma on its output.
The ADC has 100M input resistance.

Googling came up with the following suggestions:
#1)Add a low voltage reference in series to temperature sensor.

#2) Try connecting the output of the temp sensor to the ADC through a 1k
resistor. Take another 1k resistor and connect it between the reference
voltage and the ADC input. This may not work if your temp sensor will not
sink enough current.

#3) Many ADC's have voltage reference outputs that can be used to bias the
analog input(s) of the ADC. This can be done by connecting a resistor from
the ADC reference output to the analog input. The Voltage reference output
can be bypassed to analog ground with a small capacitor to improve the
ripple rejection. The bias resistor value can be selected based upon the
ADC input leakage current. Consider a resistor value such that the maximum
ADC input leakage current alone causes less than a 1 LSB voltage across the
bias resistor. In this way the "offset code" of the ADC will not be overly
input bias current dependent.

#1 seems straight forward enough, however, #2 only requires two resistors
and #3 only one.

Questions:
- Is one or is two resistors the better solution?
- #3 gives specifics on selecting the resitor value. Any help with this, I'm
lost?

thx,
t
 
P

peterken

Jan 1, 1970
0
Easiest and most accurate solution is using the "voltage divider" with two
resistors connected to the reference.
Use 0.1% resistors.

#1 requires an extra reference, including its tolerances, thus requiring
calibration
(AD has its own internal reference, thus compensating its own
measurments)
#2 would be the solution (if ADC doesn't sink enough, take for example order
of 10k)
Any arising voltage drops due to the R- network can be incalculated in
software
#3 has the disadvantage of different leakage currents for every device, thus
requiring calibration


I have a AD594 temp sensor (~10mV/C output) connected to the 10bit ADC on an
AVR. The sensor output will be -375mV to +1015mv for the temperature range
(-40C to +100C) I'm interested in. The ADC only accepts +V analog input so
I'll need to bias the signal 'up' a little to get it above 0V. If possible
the solution should be repeatble in production and not require individual
calibration.

Specs:
Using a TC7660 or TCM829 for the negative supply to the AD594.
I'm using the internal 2.56V Vref of the AVR ADC. Vref is exposed on a pin.
The AD594 can source 5ma on its output.
The ADC has 100M input resistance.

Googling came up with the following suggestions:
#1)Add a low voltage reference in series to temperature sensor.

#2) Try connecting the output of the temp sensor to the ADC through a 1k
resistor. Take another 1k resistor and connect it between the reference
voltage and the ADC input. This may not work if your temp sensor will not
sink enough current.

#3) Many ADC's have voltage reference outputs that can be used to bias the
analog input(s) of the ADC. This can be done by connecting a resistor from
the ADC reference output to the analog input. The Voltage reference output
can be bypassed to analog ground with a small capacitor to improve the
ripple rejection. The bias resistor value can be selected based upon the
ADC input leakage current. Consider a resistor value such that the maximum
ADC input leakage current alone causes less than a 1 LSB voltage across the
bias resistor. In this way the "offset code" of the ADC will not be overly
input bias current dependent.

#1 seems straight forward enough, however, #2 only requires two resistors
and #3 only one.

Questions:
- Is one or is two resistors the better solution?
- #3 gives specifics on selecting the resitor value. Any help with this, I'm
lost?

thx,
t
 
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