Design of multi-channel programmable gain amplifier circuit in ERT

Electrical Resistance Tomography (ERT) is a process tomography technology that detects resistivity information in a material field based on the principle of electrical sensitivity. This technology has the advantages of no radiation, visualization, and non-invasiveness, making ERT technology have broad application prospects in the fields of industrial process detection and medical clinical monitoring.

In the ERT system based on parallel data acquisition, since the voltage signals outputted from each measuring electrode are different, the voltage signals sent to the input of the analog-to-digital converter by each sampling channel are also different. Therefore, it is not possible to use a fixed gain amplifier circuit to amplify the signal. At the same time, since the dynamic range of the weak signal to be measured is wide, the amplifier circuit in each channel is required to make corresponding gain adjustments according to the size of the input signal, thereby improving the resolution of the entire system. In this paper, the design of a multi-channel programmable amplifier circuit is studied based on the above problems.

1 System composition of multi-channel programmable amplifier circuit

The basic composition of the sampling circuit composed of a multi-channel programmable amplifier circuit is shown in Figure 1. The entire circuit consists of an amplifier module, a gain control module, an A/D conversion module, a microcontroller, and an EEPR-OM module. The amplifier circuit amplifies the weak input signal to the appropriate range of the analog-to-digital converter.

The process of program-controlled amplification is: detect the voltage signal output from the measuring electrode to determine whether it meets the input requirements of A/D conversion. If not, the microcontroller uses the gain control module to control the gain of the amplifier circuit to meet the requirements.

2 System Hardware Design

In the hardware design of multi-channel programmable amplifier circuit, the main design is signal amplifier module, gain control module and microcontroller module. The main function of the signal amplifier module is to amplify the weak signal output by each channel; the gain control module is mainly to detect whether the amplified signal meets the input requirements of the A/D converter. If not, the signal amplifier module is controlled to perform programmable amplification; otherwise, no output changes are made. The microcontroller module mainly completes the control of the gain module.

2.1 Signal Amplification Module

In the design of the signal amplification module, the chips involved include INA128, AD603 and AD817. The design schematic is shown in Figure 2.

Since the signal measured by the ERT system is at the mV level or even smaller, and the signals are collected and amplified simultaneously in the multi-channel programmable amplifier circuit, common-mode interference is prone to occur. In order to ensure the effectiveness of the signal and remove the interference, the instrument amplifier INA128 is selected in the figure to reduce the interference. Among them, INA128 has the advantages of low input offset voltage of 50μV; low offset voltage drift of 0.5μV/℃; high input impedance; low input bias current of 5 nA; high common-mode rejection ratio of 120 dB (G=100).

The differential amplifier part of the design uses integrated operational amplifier AD603 and operational amplifier AD817 to form a two-stage amplification. Among them, AD603 is a chip with programmable gain adjustment function, so the gain control module is mainly used to program the AD603; and the circuit part of AD817 is connected to a fixed gain amplification factor.

AD603 is a patented product of ADI, a low-noise integrated operational amplifier with 90 MHz bandwidth and adjustable gain, and a slew rate of 275 V/μs. The connection method between the pins determines the programmable gain range. The bandwidth is 90 MHz when the gain is -11 to +30 dB, and the bandwidth is 9 MHz when the gain is 9 to 41 dB. Changing the connection resistance between the pins can make the gain within the above range. The gain is determined by the voltage difference provided between pin 1 and pin 2 of AD603. The gain formula is:

Among them, VG=VGPOS-VGNEG, and VC varies between -500 and +500 mV.

AD817 is a low-cost, low-power, high-speed operational amplifier that uses a single or dual power supply and features a 50 MHz unity gain bandwidth, a 350 μs slew rate, and a 45 ns 0.1% settling time. In Figure 2, the AD817 circuit is connected to a fixed gain amplifier circuit, so that it is connected to the previous stage AD603 to increase the amplifier multiple.

2.2 Gain Control Module

The gain control module mainly provides voltage for AD603 pin 1 and pin 2, and changes the gain by controlling the voltage difference between the two pins. For the programmable amplifier circuit in ERT parallel data acquisition, in order to reduce the complexity of the system to a certain extent, the voltage of pin 2 is fixed, and the voltage difference VC is changed by changing the voltage value of pin 1, thereby changing the gain G. The schematic diagram of the gain control module is shown in Figures 3 and 4.

In Figure 3, the output voltage of the D/A converter chip AD8600 is selected. When the output voltage changes, the voltage value on pin 1 of the chip AD603 in the multi-channel programmable amplifier circuit also changes. The AD8600 is a voltage output type D/A converter chip with 16 independently addressable DACs. Each DAC has its own DAC register and input register, but all DACs share a reference input voltage. The digital interface of the chip includes an 8-bit parallel data input terminal, 4 address signal lines and

Control signal lines, etc. AD8600 is an 8-bit DAC chip, and its output voltage swing is between DACGND and the external reference voltage KREF.

In order to make the voltage difference VG between pin 1 and pin 2 of AD603 change between -500 and +500 mV, and the output voltage of AD8600 is determined by its voltage reference, the voltage on pin 2 is fixed to 0.5 V, so that the input voltage on pin 1 is output between 0 and 1 V, and the reference voltage chip is ADR510 from ADI. ADB510 is a low-voltage, precision, shunt mode reference voltage source with a temperature coefficient of 70×10-6/℃, which has high precision and ultra-low noise performance.