Re: Biasing ADC input
From: peterken (peter273_at_hotmail.com)
Date: 10/25/04
- Next message: TimR: "Re: lightning rod question"
- Previous message: oddsod: "OT ?"
- In reply to: thomas: "Biasing ADC input"
- Messages sorted by: [ date ] [ thread ]
Date: Mon, 25 Oct 2004 10:30:53 GMT
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
"thomas" <none@yahoo.com> wrote in message
news:Ttaed.3156$Ag.2240@fe37.usenetserver.com...
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
- Next message: TimR: "Re: lightning rod question"
- Previous message: oddsod: "OT ?"
- In reply to: thomas: "Biasing ADC input"
- Messages sorted by: [ date ] [ thread ]
Relevant Pages
|
