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AN4348 データシート(PDF) 13 Page - STMicroelectronics

部品番号 AN4348
部品情報  Signal conditioning for electrochemical sensors
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AN4348
Other op-amp parameters to consider
DocID025166 Rev 1
13/25
9
Other op-amp parameters to consider
Consumption is the most critical parameter for devices that are battery powered. However,
if the application is aggressive in terms of accuracy, the input/offset voltage (Vio) of the op-
amp should also be considered even in the case of a potentiostat. In fact, the Vio of a
potentiostat is subtracted from the output signal. For example, a Vio of 3 mV means an
error of 0.3 ppm for a NO2 sensor with a sensitivity of 100 nA/ppm used with an RT of 100
kΩ. Vio also impacts startup time. Most sensors are highly capacitive and are in the range
of 100 mF. At startup, once the output is no longer saturated, the sensor shifts the charge
to the Vio with a constant time of RTC (tens of seconds).
To operate correctly, the potentiostat configuration requires that the input bias current (Iib)
of the op-amp is low to prevent some current flowing into the reference electrode (see U2
in Figure 7), and to avoid an additional offset caused by RT. The accuracy of the application
does not generally require the Iib to be lower than 1 nA. Therefore, any CMOS or JFET op-
amp is a good candidate in this respect.
Both Vio and the Iib can be compensated by calibration. As long as the system remains
linear, the output signal read by the ADC is in the form Req × Isense + Voffset. The output
signal depends on the sensitivity of the sensor, RT accuracy, Vref, Vref2, Iib, and the Vio of
U2. By measuring the gas at two different concentrations, one can determine the equation
to set in the microcontroller to retrieve the gas concentration.
However, even if Vio and Iib can be compensated, their variation with temperature cannot.
Therefore, for very high accuracy applications, the TSZ121, whose temperature coefficient
is only 0.06 µV/˚C maximum, is perfect. With this op-amp, a 30 ˚C variation causes a shift
of only 1.8 µV which is clearly much lower than the resolution of a 12-bit ADC powered by
3.3 V.
Note that any change in Vref2 can be monitored with an additional ADC channel.
As shown in Table 1, the TSU10x, TSV71x, TSZ12x, and TSV73x are very good CMOS
op-amps that fulfill the requirements of electrochemical sensing-based applications.
Table 1: Op-amp selection specification at 3.3 V
Product
GBP
Icc (per channel)
TSU101
TSU102
TSU104
8 kHz
600 nA
TSV711
TSV712
TSV714
120 kHz
9 µA
TSZ121
TSZ122
TSZ124
400 kHz
29 µA
TSV731
TSV732
TSV734
850 kHz
59 µA


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