Design of Reference Source Based on AD9852

High-precision programmable constant current power supplies are widely used in instrumentation, sensor technology and testing. In the past, most programmable constant current source circuits used PWM pulse mode, which is easy to control and adjust, but the accuracy is difficult to guarantee, and the waveform duty cycle adjustment range of the PWM mode is limited, which is difficult to meet the requirements of continuous adjustable large current. This article introduces a solution that uses STC89C52 single-chip microcomputer to control the voltage-controlled constant current source and realizes the program control of the constant current source through the current expansion circuit, and its output current value can reach 2A.

The composition and working principle of the programmable constant current source
The programmable constant current source circuit consists of a voltage control circuit, a current expansion circuit and a digital control circuit, and the structure is shown in Figure 1.

Figure 1 Block diagram of programmable constant current source circuit

This constant current source circuit uses STC89C52 to control the D/A conversion circuit to generate a voltage control signal, and outputs the required current value through a precision linear voltage-controlled current source and a current expansion circuit; the sampling circuit samples and reads out the current through the A/D conversion, and then sends it to the display control circuit for display; at the same time, the sampling circuit provides current negative feedback to the voltage-controlled current source to further stabilize the current output.

Design of program-controlled constant current source circuit
1 Design of digital control circuit
The digital control circuit uses a single-chip minimum system composed of STC89C52 to control D/A, A/D, as well as key response and LED display. The digital circuit and analog circuit in the module are each powered by an independent voltage regulator circuit to reduce the impact of the high-frequency peak current of the digital circuit on the analog circuit, which can greatly reduce the ripple voltage of the D/A output.

The D/A conversion circuit in this design adopts MAX531 and its internal 2.048V reference source. The resolution of D/A conversion is 0.5mV. When added to a 1Ω sampling resistor, it can distinguish 0.5mA current (step 0.5mA).

The A/D conversion circuit uses MAX1241, which uses the same reference source as MAX531. The resolution of A/D conversion is 0.5mV. When the sampling resistance is 1Ω, the resolution of the measured current is 0.5mA (the number of bits and reference voltage of D/A and A/D converters can be selected according to the actual requirements of stepping and measurement accuracy).

To realize human-computer dialogue, at least 10 numeric keys and 2 stepping keys are required. Considering that other function keys need to be realized, a 16-key keyboard is the most suitable to complete the whole system control. The display part uses 8-bit LED digital tube, which is cheap and easy to realize. Considering the limited I/O port of the single-chip microcomputer, in order to fully optimize the system, an external expansion 8155 is used to realize the keyboard interface and display function.

2 Design of voltage-controlled current source
The negative feedback amplification part of the voltage-controlled current source has a common-phase amplifier composed of a precision op amp, which introduces deep current negative feedback to stabilize the current output to the load, as shown in Figure 2. When the op amp works normally in the common-phase amplification state, the voltage on the sampling resistor can be known from the principle of virtual ground of the op amp: U2=Uin, so I2=U2/R2=Uin/R2. Because of the high input impedance amplifier, the current at the inverting input terminal is approximately zero, and the load current IL=I2=Uin/R2. As long as the performance of the current expansion circuit is good, the accuracy of the output current depends entirely on the accuracy of the sampling resistor.

Figure 2 Schematic diagram of voltage-controlled current source circuit [page]

3 Design of current expansion circuit
The current expansion circuit uses a class S power amplifier, and the principle is shown in Figure 3. Its characteristic is that a voltage control amplifier and a current drive amplifier form a bridge, so that the voltage amplifier works in a no-load state (output current is zero), and the back stage works in a voltage-controlled follower state, which can easily achieve a good following effect. For the load, the front and back stages are output in parallel, and the negative feedback is derived from the sampling resistor and sent back to the front stage amplifier. Therefore, the quality of the class S power amplifier depends on the front stage.

Figure 3 Schematic diagram of the current expansion circuit of Class S power amplifier

The core of the Class S power amplifier circuit is a current drive amplifier with strong load capacity, which is coupled to the load through a bridge. Assuming that the open-loop gain of the amplifier is close to infinity, the voltages at the two input terminals of the amplifier will be extremely close, which can be expressed by the formula: I1R1=I2R2, I3R3=I4R4.

If the input impedance of the amplifier is infinite and the current at the two input terminals of the amplifier is approximately zero, then I2=I4, and I1=I2R2R3/R4R1; when the bridge is balanced, R2R3=R4R1, so I1=I2, and therefore I1=0.

According to the above derivation, when the S-class power amplifier circuit works stably, the front-stage amplifier circuit works in a no-load or light-load state, and the current required by the load is completely provided by the rear-stage current drive amplifier circuit. In this way, the circuit does not have high requirements on the load of the front-stage voltage-controlled current source.

In summary, as long as you choose a post-stage power amplifier chip with high input performance and strong load capacity, the output change is completely determined by the pre-stage. When the pre-stage works in a no-load state, its performance is basically independent of the load change. In this way, when designing the pre-stage, you can put aside the consideration of load capacity and directly use a high-precision, low-offset operational amplifier; when designing the post-stage, because the output depends on the pre-stage, there is no need to worry that the addition of the load will affect its working performance, and the selection range becomes wider.

Based on the design principle of Class S power amplifier circuit, in order to ensure the reliability and sufficient performance of the circuit, a high-quality power amplifier chip LM3886 is used, whose electrical performance is very close to that of an ideal amplifier and has sufficient output power.

The test results show that no matter it is a large current or a small current, the change in load resistance has little effect on the system, indicating that the system meets the basic requirement of constant current.

Conclusion
The main feature of this programmable constant current source is that it adopts S-type feedback control amplifier circuit to achieve precise current control, and has the advantages of convenient operation, stability and reliability, and has excellent performance through actual testing.