OA1ZHA Datasheet, PDF (18/35 Pages) STMicroelectronics – Fitness and healthcare

Application information
OA1ZHA, OA2ZHA, OA4ZHA
In the time domain, the offset part of the output signal before filtering is shown in Figure 40:
"Vio cancellation principle".
Figure 40: Vio cancellation principle
5.1.2
The low pass filter averages the output value resulting in the cancellation of the Vio offset.
The 1/f noise can be considered as an offset in low frequency and it is canceled like the Vio,
thanks to the chopper technique.
Frequency domain
The frequency domain gives a more accurate vision of chopper-stabilized amplifier
architecture.
Figure 41: Block diagram in the frequency domain
The modulation technique transposes the signal to a higher frequency where there is no 1/f
noise, and demodulate it back after amplification.
1. According to Figure 41: "Block diagram in the frequency domain", the input signal Vin
is modulated once (Chop1) so all the input signal is transposed to the high frequency
domain.
2. The amplifier adds its own error (Vio (output offset voltage) + the noise Vn (1/f noise))
to this modulated signal.
3. This signal is then demodulated (Chop2), but since the noise and the offset are
modulated only once, they are transposed to the high frequency, leaving the output
signal of the amplifier without any offset and low frequency noise. Consequently, the
input signal is amplified with a very low offset and 1/f noise.
4. To get rid of the high frequency part of the output signal (which is useless) a low pass
filter is implemented.
To further suppress the remaining ripple down to a desired level, another low pass filter
may be added externally on the output of the OA1ZHA, OA2ZHA and OA4ZHA device.
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