Realizing a High-Precision Programmable Gain Amplifier Using DA Converter

Realizing a High-Precision Programmable Gain Amplifier Using D/A Converter

In practical applications, a gain-programmable amplifier (PGA) is often required to amplify analog input signals of different amplitudes to a certain range, which is convenient for sampling by the A/D converter, or to amplify a given signal by a gain set by the software and then output it. However, there are not many ready-made programmable gain amplifiers available for selection, and other methods need to be used to achieve it. There are usually two methods: 1) op amp + analog switch + resistor network; 2) op amp + digital potentiometer. Among them, the former method uses an analog switch to switch the resistor feedback network to change the closed-loop gain of the amplifier circuit. This method requires more components, the circuit is large, and the accuracy is limited. The second solution uses a solid-state digital potentiometer to control the gain of the amplifier circuit, and the circuit is relatively simple. However, the existing digital potentiometers have limited resolution, and the common ones are 32 and 64 taps, and a few can reach 1024 taps. Therefore, the precision of the amplifier is limited and cannot meet the requirements of 10-bit or even 12-bit data acquisition systems.

    In fact, there is another method that is simpler and more accurate, but often overlooked. That is to use a D/A converter to realize a high-precision programmable gain amplifier. Figure 1 shows a 12-bit programmable gain amplifier using MAXIM's 12-bit D/A converter MAX502. It can be seen that this solution is very simple. Only a single-chip D/A converter is needed to realize a complete high-precision PGA, and even no peripheral components are required (the fine-tuning potentiometers W1 and W2 in the figure can be omitted). And it also has a very convenient programming interface, which can be directly connected to the data bus (16-bit bus can use MAX502, 8-bit bus can use MAX501, SPI serial bus can use MAX532). The following is a brief analysis of its working principle and performance parameters.

    How it works

    The MAX502 D/A converter uses an R-2R ladder decoding network to achieve the conversion from digital to analog     . By inputting different digital quantities D, the voltage gain of the amplifier can be set between 1 and 4096. The two external potentiometers W1 and W2 in Figure 1 are used to fine-tune the offset and gain errors of the amplifier, which can further improve the accuracy of the amplifier and can be omitted if the requirements are not too high.
    

    Performance parameters

    The performance parameters of the amplifier can be converted according to the corresponding parameters of the D/A converter. According to the performance indicators of MAX502, the main performance parameters of the circuit in Figure 1 are as follows:

      Input offset voltage: <±1mV (unadjusted)
      Input offset temperature drift: ±5μV/℃
      Gain error: <0.07% (unadjusted)
      Gain temperature coefficient: <±1ppm/℃
      Unit gain bandwidth: 3MHz
      Slew rate: 5V/μs
      Output swing: ±10V (2kΩ load)
      Output resistance: 0.2Ω
      Output noise voltage: 25nV/Hz
      Total harmonic distortion (THD): -90dB