Offset compensation for high-gain AC amplifiers

DC feedback-coupling can provide an easy method of cancelling DC offset from high-gain AC amplifiers in PSoC applications.

Programmable System-on-Chips (PSoC™) are often used in signal conditioning and processing applications, however, signals from microphones, photodiodes, magnetic heads or PIR sensors can be weak and may need to be amplified using a Programmable Gain Amplifier (PGA) module. Problems arise, however, when the desired gain is greater than 100. This problem is caused by the PSoC internal amplifier input offset voltage, which may be as much as 10mV. The amplified offset voltage distorts the symmetry of the AC output signal and decreases the dynamic range of successive signal-processing stages such as the ADC. The generic solution to this problem is to use an additional RC circuit between two PGAs (see Figure 1).

 

 

The disadvantage of this generic solution is that it requires several PSoC analogue I/O pins for the analogue ground (AGND) output and for the RC circuit between PGAs. To rectify the disadvantage, this circuit can be modified by adding DC feedback coupling between the output and input, which frees up the I/O pins for other purposes.

 

 

Amplifier with DC feedback

This circuit uses only two analogue I/O pins (instead of the four in the generic solution). PGA2 is replaced by an Inverting Amplifier (AMPINV) user module to achieve inverted output as opposed to input. This allows the connection of output to input via the additional RC circuit to form negative feedback coupling. Capacitor C2 suppresses the AC component of the feedback signal. The DC component passes from output to input without any changes, which forms 100% DC negative feedback. This permits the output DC voltage to be close to analogue ground, independent of the amplifier’s input offset voltage. The circuit in Figure 2 seems simple, but it is not easy to estimate the gainfrequency characteristic of the whole amplifier due to frequency-dependent feedback. For analysis purposes, the amplifier circuit should be slightly modified as shown in Figure 3.

 

 

The resistor R1 is added, taking into account the AC signal source output resistance. PGA and AMPINV coupled in series are replaced by one equivalent inverting amplifier with gain equal to k. Using Kirchhoff’s laws and simple algebra, the frequency-response function for the circuit shown in Figure 3 can be written as follows:

 

 

The above equation describes a combination of high-pass and band-pass filters with the same roll-off frequency, calculated as:

 

 

The gain at high frequencies is equal to:

 

 

There are no restrictions for selecting component values. Even so, the roll-off frequency should be set relatively low, less than 1-2kHz in order to ensure stability of the circuit and to prevent a resonant peak, R4 should be increased.

 

DC feedback via modulator

The amplifier with feedback that is shown in Figure 2 uses two analogue I/O pins for input and output. Occasionally, all analogue pins are required for signal input purposes. In this case, the digital I/O pin can be used to form the feedback loop. The analogue signal is transferred via a digital pin using the Analogue-to-Digital (AD) modulator on the type C switched-capacitor block. The analogue signal from the PGA output is routed to the AD modulator. The modulator converts the analogue signal into a series of digital pulses. The average value of these digital pulses is equal to the analogue input signal.

The AD modulator produces digital pulses on the corresponding analogue column comparator bus. To route this digital signal to the output pin, the Digital Buffer (DigiBuf ) user module is used. For the PSoC CY8C24x94 device family, the comparator bus can directly drive the global output bus. If the digital signal is used for feedback, then inversion can be performed by either the INVAMP or the digital row LUT.

The incremental and sigma-delta ADC use the analogue modulator as a one-bit, analogue-todigital converter. Therefore, the ADC’s modulator can be used to form the compensation feedback signal by routing the ADC’s comparator bus signal to the digital buffer input.

 

Conclusion

Described in this note is an AC amplifier with feedback that can not only be used to cancel DC offset, but also as an active hardware filter. This is due to its transfer function which corresponds to the second-order analogue filter. By modifying the feedback chain, the frequency-gain characteristic can be modified to form another filter type, for example, a bandpass filter. These solutions can be tested using a Cypress CY3210 PSoCEVAL1 kit and PSoC Designer v4.4 software. An extended version of this article and a downloadable PC application are available.

 

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