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AN4348 データシート(PDF) 13 Page - STMicroelectronics |
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13 / 25 page 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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同様の説明 - AN4348 |
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