What is PGA?

With the application of computers, in order to reduce hardware equipment, a programmable gain amplifier (PGA) can be used. It is a very versatile amplifier, and its amplification factor can be controlled by program as needed. With this amplifier, the amplification factor can be adjusted by program to make the full-scale signal of the A/D converter uniform, thereby greatly improving the measurement accuracy. The so-called automatic range conversion is to use the programmable gain amplifier to automatically adjust the multiple of the processed signal according to the needs to meet the requirements of subsequent circuits and systems. There are two types of programmable gain amplifiers - combined PGA and integrated PGA.

Combined PGA

The combined PGA is generally composed of an operational amplifier, an instrument amplifier or an isolated discharger plus some other additional circuits. The working principle of the device is to adjust the value of the feedback resistor connected to the multiplexer through the program, thereby adjusting the amplifier's gain.

The commonly used instrument measurement amplifier adopts a two-stage amplifier circuit. The first stage adopts a common-direction parallel differential amplifier, and the second stage adds a basic differential amplifier to form an instrument amplifier. The biggest advantage of this circuit is high input impedance, strong common-mode suppression capability, convenient gain adjustment, and because of the symmetrical structure, small vector modulation voltage and temperature drift, it is widely used in the sensor weak signal amplification system.

Integrated PGA

A specially designed programmable gain amplifier circuit is an integrated PGA. There are many types of integrated PGA circuits, such as the MCP6S21, MCP6S22, MCP6S26, and MCP6S28 series produced by Microchip, and the AD8321 produced by Analog Devices, all of which are programmable gain amplifiers. The following uses the MCP6S series PGA as an example to illustrate the principle and application of this circuit, and the others are similar.

The MCP6S series is a single-ended, cascadeable, gain-programmable amplifier. The MCP6S21, MCP6S22, MCP6S26, and MCP6S28 are 1-channel, 2-channel, 6-channel, and 8-channel programmable gain amplifiers, respectively. Their main features are as follows:

8 programmable gain options: +1, +2, +4, +5, +8, +10, +16 or +32;

SPI serial programming interface;

Cascade input and output;

Low gain error, maximum plus or minus one percent;

Low drift, maximum ±275uv;

Low supply current, typically 1mA;

·Single power supply, 2.5V~5.5V.