MCP6491 Datasheet, PDF (19/50 Pages) Microchip Technology – 7.5 MHz, Low-Input Bias Current Op Amps

4.7.3 PH ELECTRODE AMPLIFIER
The MCP6491/2/4 op amps can be used for sensing
applications where the sensor has high output
impedance, such as a pH electrode sensor; its output
impedance is in the range of 1 M to 1G. The key op
amp specifications for these kinds of applications are
low-input bias current and high-input impedance.
A typical sensing circuit is shown in Figure 4-12, it is
implemented with a non-inverting amplifier which has a
gain of 1+R₂/R₁. The input voltage error due to input
bias current is equal to I_B*R_OUT, which is amplified by
1+R₂/R₁ at the output. To minimize the voltage error
and get the V_OUT with better accuracy, the I_B must be
small enough.
R₂
R₁
–
MCP6491
V_IN
+
R_OUT
V_SEN
+
–
pH electrode
V_OUT
V_SEN is the sensed voltage by pH electrode
R_OUT is the pH electrode’s output impedance
FIGURE 4-12:
pH Electrode Amplifier.
MCP6491/2/4
 2012-2013 Microchip Technology Inc.
DS20002321C-page 19