AN4348 datasheet(13/25 Pages) STMICROELECTRONICS

13 / 25 page

AN4348

Other op-amp parameters to consider

DocID025166 Rev 1

13/25

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

AN4348 数据表(PDF) 13 Page - STMicroelectronics

AN4348 数据表(HTML) 13 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

数据表下载