On 2006-08-02, Z wrote:
Z wrote in :

No, the circuit I was asking about is a buffer/gain/offset for a pH
sensor. The pH sensor is a battery that actually puts out a positive
voltage when its below neutral (acidic) and a negative voltage when it
is above neutral (base). My ADC is single supply so it cannot handle the
negative voltage.

I'm using a 2.5V reference for the ADC and the pH sensor will never put
out more than +/-450MV so I went with 2.5 times gain (+/- 1.125) and an
offset that would put neutral (zero from sensor) smack dab in the middle
of the ADC range. with the offset that would give an ADC input between
.125V and 2.375V.

The range I'm actually interested in is pH7 to pH8 and I need two
decimal places, so I'm using a 12 bit ADC. I would use a higher gain to
have 6-9 PH span the entire ADC range, but I cannot figure out how to
clamp the ADC input at zero volts. The ADC can withstand -0.5V on an
input and I considered using a small signal diode from the ADC input to
ground, but the voltage drop is larger than the -0.5V limit on the
input.

use a germanium small-signal diode: 0.2V,
or a schottky diode: 0.3V

The three amp and two amp versions work perfect but I want to learn how
to do it with one amp!

what's the output impedance of the pH sensor.
is the output ground referenced, isolated, or differential?

Bye.
Jasen