Offset Compensation for High Gain AC Amplifiers

By Vadym Grygorenko, Sr. Application Engineer, Cypress Semiconductor

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A simple solution is proposed to cancel DC offset in high gain AC amplifiers. This solution is based on external DC feedback-coupling. Included with this Application Note is the supporting PC application to estimate amplifier frequency response results.

Introduction

The typical PSoC™ device application includes signal conditioning and processing from various sources. Often, a weak AC signal needs processing. That signal can be from a microphone, photodiode, magnetic head, PIR sensor and so on. A Programmable Gain Amplifier (PGA) User Module is commonly used for weak signal conditioning. Problems arise 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 (ADC, etc.). 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 it requires several PSoC analog I/O pins for analog 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

The modified AC amplifier circuit is shown in Figure 2.

 

 

This circuit uses only two analog 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 analog ground, independent of the amplifier’s input offset voltage.

The circuit in Figure 2 seems to be very simple. Nevertheless, it is not easy to estimate the gain-frequency 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:

Equation 1 describes a combination of high-pass and band-pass filters with the same roll-off frequency.

In order to facilitate the estimation of frequency-gain response characteristics at middle frequencies, a simple PC application was developed. This application tabulates and plots a frequency response curve according to Equation 1. The interface is shown in Figure 4.

 

 

To estimate frequency-response characteristics, the component values are entered and the “Calculate” button clicked. All resistances are expressed in kilohms, capacitors in microfarads. The “Gain” value is equal to the product of the gains of the PGA and AMPINV (see Figure 2).

There are no restrictions for selecting component values. Even so, the roll-off frequency, (Equation 2), should be set relatively low, less than 1-2kHz in order to ensure stability of the circuit. To decrease the resonant peak on the frequency plot, R₄ should be increased.

 

DC Feedback via Modulator

The amplifier with feedback that is shown in Figure 2 uses two analog I/O pins for input and output. Occasionally, all analog pins are required for signal input purposes. In this case, the digital I/O pin can be used to form the feedback loop. The analog signal is transferred via a digital pin using the analog-to-digital modulator on the type C switched capacitor block. For details on modulator functions, refer to Application Note AN2041 “Understanding Switched Capacitor Analog Blocks”. The subsequent device block schematic is shown in Figure 5.

 

 

The analog signal from the PGA output is routed to the AD modulator. The modulator converts the analog signal into a series of digital pulses. The average value of these digital pulses is equal to the analog input signal. The SCBlock User Module parameters to form the AD modulator are shown in Figure 6.

 

 

The AD modulator produces digital pulses on the corresponding analog 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 without the Digital Buffer using the Comparator Bus to Global Outputs Enable register (CMP_GO_EN).

If the digital signal is used for feedback, then inversion can be performed by either the INVAMP or the digital row LUT (see Figure 7).

 

 

Important Tip

The incremental and sigma-delta ADC use the analog modulator as a one-bit, analog-to-digital converter. Therefore, the ADC’s modulator can be used to form the compensation feedback signal. You just need to route 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, (Equation 1), which corresponds to the second order analog filter. By modifying the feedback chain, the frequency-gain characteristic can be modified to form another filter type, for example, a band-pass filter.

 

 

Test This Programmable Solution for Yourself with a Cypress CY3210-PSoCEVAL1 Kit and PSoC Designer v4.4 Software

This evaluation kit features an evaluation board and MiniProg programming unit. The evaluation board includes an LCD module, potentiometer, LEDs and plenty of breadboarding space to meet all of your evaluation needs. The MiniProg programming unit is also included with the kit and will program PSoC devices directly on the evaluation board, or on other boards via a 5-pin header. This programming unit is small and compact, and connects to a PC via a provided USB 2.0 cable.

Kit includes

• Evaluation board with LCD module
• MiniProg programming unit
• PSoC Designer Software CD
• 28-pin CY8C29466-24PXI PDIP PSoC device sample
• USB 2.0 cable
• Getting Started Guide

PSoC Designer software helps users harness the power and flexibility of PSoC devices by providing “drag and drop” system design capability inside a full featured development environment with C Compiler and Assembler. This powerful easy-to-use integrated development environment (IDE) for PSoC applications comes with over 50 pre-configured, pre-characterized embedded peripheral functions called user modules, including the following programmable analog and digital functions:

 

ANALOG ADC Incremental 6– 14 bits Delta Sigma 6–13 bits DAC 6, 8 and 9-bit 6 and 8-bit multiplying Filters 2-pole low-pass filter 2-pole band-pass filter Notch filters DTMF dialer Modulator Peak detector V to I converter Amplifiers Programmable gain amplifier Instrumentation amplifier Inverting amplifier Comparators Programmable comparator Hysteresis comparator Zero-Crossing comparator CapSense

DIGITAL 8, 16, 24, 32-bit timer 8, 16, 24, 32-bit counter 8, 16, 24, 32-bit PWM 8, 16-bit dead band generator Pseudo random source Cyclic redundancy check     COMMUNICATIONS INTERFACE I2C Master I2C Slave SPI Master SPI Slave Full Duplex UART Tx, Rx Full Speed USB V2.0

 

Upgrade to real mixed-signal programmability today with the revolutionary Cypress PSoC mixed-signal array, a complete system-level solution offering

• The integration of an ASIC
• The flexibility of an FPGA
• The industry’s easiest system-design software tools

 

 

“MiniProg” Programmer

The MiniProg provides a simple to use and cost effective programming solution that is superb for a development environment. It consists of a USB interface to the development PC and a 5-pin header that is used to connect to the target board. It works in conjunction with the GenProg software available through download as part of the PSoC Designer tools on the Cypress website. The MiniProg can also be used in conjunction with the CY3216 to program devices out-of-system, or with the CY3655-PLG to program enCoRe II devices through the USB connector.

The CY3217 kit consists of the same programmer that is available in the PSoC CY3210-MiniProg1, but provided as a stand-alone item without the PSoC evaluation board. This kit is an ideal item to enable quick turn programming at multiple development/test sites.

The CY3217 “MiniProg” USB Programmer includes:

• (1) MiniProg programmer board
• (2) Std-A to Mini-B USB cable

 

 

I²C to USB Bridge Debugging/Communication Kit

The I²C to USB Bridge is a quick and easy link from any design or application’s I²C bus to a PC via USB for design testing, debugging and communication. The bridge is based on 1 of Cypress’ CY8C24894, which contains 6 analog blocks supporting ADCs, DACs and filters, 4 digital blocks supporting PWMs, timers and counters and a full-speed USB 2.0 peripheral.

 

 

 Featured Products

Part Number		Description	Data Sheet	App. Notes
CY3210-PSOCEVAL1		PSoC Development Kit
CY3217		"MiniProg" Programmer Kit
CY3240-I2USB		I2C to USB Bridge Debugging/Communication Kit

refers to New Product Introduction