Part Number:LMP7721
Hi TI engineers!
(I expect to be answered by Paul Grohe, as he was responsible for the design of the above mentioned circuit board, however, it isn't necessary to)
I'm evaluating the LMP7721 eval. board to see if it fits for my application, which is just read a pH probe sensor. For this application I need a very high input impedance and level-shift the signal, as the pH probe works in a range of approx. +-500mV and I will use single supply (+5V and 0V). Also, I don't use TRIAX conector, just a normal BNC, i.e., guard is necessary but not connected to the BNC plug.
Here comes my question: Regarding the document "LMP7721 Multi-Function Evaluation Board Users' Guide", page 5, section 4.1.1, Figure 2 shows the "Non-Inverting Configuration (Factory Configuration)", which seems to be the best for my job. However, if we modify this circuit for single power supply instead of dual, and therefore, change the guard level from GND to Vbias by connecting pads 1 and 2 instead of 2 and 3 of RFb in Figure 9, pg 13, the Vbias voltage will appear in the output just as: Vout = -RFa/RFb x Vbias, i.e., the offset voltage appears inverted (negative) in the output! But it doesn't make sense as I will use a single power supply. Am I doing something wrong?
Is this board suppose to work with single supply and Vbias applied to the guard?
Regarding the second configuration shown in section 4.1.2 "Inverting configuration", page 6, this configuration seems to fix the above mentioned problem, however, the input impedance for this configuration (Figure 3) is about Rs+R2, i.e., about 1MOhm (Am I correct?). So, it's not such high impedance.. how can it work for a pH probe, for example, that as output impedance range of about 10MOhm to 1GOhm?
Looking forward to get a reply!
Thanks :)
Best Regards,
Luciano Caldas