F
Fox one! Fox one!
- Jan 1, 1970
- 0
Hi,
I am trying to debug the first stage of a pH input circuit. The first stage
is just a voltage follower made from a TL084
The pH probe centre electrode is connected directly to the non-inverting
input of one opamp of a TL084 (this connection does not touch the PCB, it's
all in mid air)
The pH probe centre electrode also has a 33pF ceramic capacitor to ground,
close to the TL084
The pH probe outer electrode is connected to ground
There is a 1M feedback resistor from the output of the opamp to the
inverting input.
When I connect a pH probe, the output of this first stage is about 75mV.
According to the TL084 datasheet it has an input offset voltage of 6mV so I
can't understand where the 75mV comes from . I have tried 2 different pH
probes, one old one new, from different manufacturers, one with pointy tip
and one with round bulb, but they both give the same result so I don't think
it's the ph sensor.
The power supply I think is my problem, I decided to use the +10V and -10V
from a MAX232 rather than use a chip like a ICL7660. The MAX232 can only
supply the +V supply of the TL084 with 5.77V and the -V supply with -3.88V.
I am trying to calculate whether this is the problem. With this power
supply, the 'ground' for the opamp is actually at 0.945V relative to circuit
ground. With the pH probe in a pH 7 calibration solution, the output
between electrodes should be close to 0V, so in this case from the opamp's
point of view both inputs are at -0.945V, and the only signal on the output
should be the offset voltage + (common mode voltage * CMRR).
The CMRR on the datasheet is 86dB, or 1 / 20,000. If the common mode
voltage on the inputs is 0.945V the contribution to the output should only
be 47uV i.e. nothing.
So, why is the first stage giving 75mV? Should I use the supply-voltage
rejection ratio instead? It's also 86dB so adding them together still
doesn't explain it.
The output of the first stage is connected via a 510K resistor to the next
stage so it is hard to imagine that that is affecting it.
Any ideas would be much appreciated.
I am trying to debug the first stage of a pH input circuit. The first stage
is just a voltage follower made from a TL084
The pH probe centre electrode is connected directly to the non-inverting
input of one opamp of a TL084 (this connection does not touch the PCB, it's
all in mid air)
The pH probe centre electrode also has a 33pF ceramic capacitor to ground,
close to the TL084
The pH probe outer electrode is connected to ground
There is a 1M feedback resistor from the output of the opamp to the
inverting input.
When I connect a pH probe, the output of this first stage is about 75mV.
According to the TL084 datasheet it has an input offset voltage of 6mV so I
can't understand where the 75mV comes from . I have tried 2 different pH
probes, one old one new, from different manufacturers, one with pointy tip
and one with round bulb, but they both give the same result so I don't think
it's the ph sensor.
The power supply I think is my problem, I decided to use the +10V and -10V
from a MAX232 rather than use a chip like a ICL7660. The MAX232 can only
supply the +V supply of the TL084 with 5.77V and the -V supply with -3.88V.
I am trying to calculate whether this is the problem. With this power
supply, the 'ground' for the opamp is actually at 0.945V relative to circuit
ground. With the pH probe in a pH 7 calibration solution, the output
between electrodes should be close to 0V, so in this case from the opamp's
point of view both inputs are at -0.945V, and the only signal on the output
should be the offset voltage + (common mode voltage * CMRR).
The CMRR on the datasheet is 86dB, or 1 / 20,000. If the common mode
voltage on the inputs is 0.945V the contribution to the output should only
be 47uV i.e. nothing.
So, why is the first stage giving 75mV? Should I use the supply-voltage
rejection ratio instead? It's also 86dB so adding them together still
doesn't explain it.
The output of the first stage is connected via a 510K resistor to the next
stage so it is hard to imagine that that is affecting it.
Any ideas would be much appreciated.