Recent studies of some typical current source circuit. Reading the classic several times the current source circuit design application manual. Combined with work experience in he sorted out to share it to all of you. Strive for in this series of documents, the common analysis of the current source circuit.
¡¾ 1 ¡¿ howland current source circuit current source circuit (a)
The first section, from the first I had recently began a howland circuit design. A project requirement for the output voltage signal change + / - 50 ma alternating current signal, so that the long distance transport. Lose the signal for the valid values for + / - 5 v of 50 hz ac voltage signal. Keep within 1% error signal. Power supply with + / - 12 v to 15 v + / can. This circuit can be designed with discrete device, using op-amp drive a class AB amplifier. The output end of the class AB amplifier series resistance as a sampling resistance. Current flows through the sampling resistance formation of the voltage signal is the feedback signal. This design has prominent shortcomings, using discrete device is more, the output device is too complex, also is not easy to control precision. So this article recommends a simpler circuit - howland current source circuit. Howland, the basic principle of the circuit diagram is as follows: the ideal op-amp X1G said.
According to the virtual short and virtual fault characteristics of the ideal op-amp we can deduce the howland circuit output current and input voltage VP, VM relation formula is as follows: [2]
We usually make the RX in the actual circuit design = RF and RZ = RI. So the above formula can be simplified as:
Circuit of the input signal is single polarity voltage signal, can put the VM grounding, the voltage is 0. This can further simplify formula to determine the value of each resistor.
Current source circuit [2] howland current source circuit (2)
Howland circuit design principle. Design the diagram below:
Circuit selects the biggest can output 200 ma current op-amp OPA551. Using single op-amp to realize the high precision of plus or minus current output. The biggest imbalance voltage of 5 mv OPA551. The biggest input signal and the design requirements of 7.071 V. So the effect of offset voltage will be less than 0.1%. Four high precision resistor R5 - R8 electric current flows through the form of voltage Number as a feedback signal feedback to R4 is input resistance. Using four is the cause of resistance, the four resistance can share the output current, ensure that heat is far lower than the rated power of resistance; Moreover, so that we can match the more precise measurement. In the implementation in production, also can reduce the resistance of random error. In order to guarantee the accuracy, R1 and R4 resistance is recommended to choose 0.1% (one over one thousand) the precision of resistance value. C1 is used to limit circuit bandwidth, at the same time to filter high frequency noise reduction.
Next, we use the TI free simulation software TINA carries on the simulation. The simulation results are as follows: from the simulation results in the coming year, the design error is less than 0.2%.
This circuit can be further improved design, as the chart, the feedback voltage signal through a high precision op-amp OPA188 as following feedback to R4. This ensures that won't be affected by the input signal of feedback signals. In the diagram, U1, U2 are negative feedback is introduced, the former phase summation operation circuit, which constitute a voltage follower.
In the actual process of plate test, the accuracy will be slightly influence, therefore recommended in PCB production, according to the actual circuit, fine-tuning the feedback resistor values to ensure accuracy. In order to achieve a higher precision.
