Home  Product Technical Articles   AD620 and PIC10F206 Based Pointer Galvanometer Design

AD620 and PIC10F206 Based Pointer Galvanometer Design

Author: Iggy
Date: 8 Mar 2022
 1867
ad620 amplifier

I. Introduction to Galvanometer

 

The galvanometer is an important testing instrument. It is a high-sensitivity mechanical indicating meter for detecting weak electricity. It is used as a zero-pointing instrument in bridges and potentiometers. It can also be used to measure weak currents, voltages, and charges.  According to the input mode, it can be divided into voltage type and current type. Generally, voltage type is more commonly used. From the panel display mode of the galvanometer, it can be divided into three types: pointer type, digital type, and mixed type.

 

The glavanometer

Figure 1 The galvanometer

 

The pointer galvanometer can conveniently observe the continuously changing current, and intuitively judge the direction of the current from the deflection direction, so it has its unique advantages in the bridge experiment. The pointer galvanometers currently used in physical experiments have the following shortcomings:

 

(1) Use 9V laminated battery with small capacity, short battery life and high cost;

 

(2) Its internal amplifying circuit  adopts OP07 or ICL7650 design, without power management capability, it is easy to cause the battery to be consumed unnecessarily;

 

(3) Due to the use of many discrete components, the amplifier is prone to drift and unstable.

 

This paper uses a galvanometer designed based on the three-op-amp high common-mode rejection ratio and high stability instrumentation amplifier AD620 and Microchip's 6-pin MCU PIC10F206 to solve the above problems.

Catalog

I. Introduction to Galvanometer

II. Hardware System Design

2.1 Hardware System Block Diagram

2.2 Instrumentation Amplifier AD620

2.3 Microcontroller PIC10F206

2.4 Amplifying Circuit of Galvanometer

2.5 Power Monitoring and Power Management Circuit of Galvanometer

III. Software System

IV. Conclusion

FAQ

Ordering & Quantity

II. Hardware System Design

 

2.1 Hardware System Block Diagram

 

The block diagram of the hardware system is shown in Figure 2. The DC voltage signal first passes through an anti-radio frequency interference low-pass filter circuit, weakens the interference signal, and then sends it to the instrumentation amplifier AD620 for differential amplification, and then drives the pointer meter to display. The 6-pin microcontroller PIC10F206 is responsible for the power monitoring and power management of the galvanometer. The whole galvanometer only uses a 3V DC power supply connected in series with two AA batteries.

 

Figure 2 The hardware system block diagram of the galvanomete

Figure 2 The hardware system block diagram of the galvanometer

2.2 Instrumentation Amplifier AD620

 

AD620 is a low-cost and high-precision instrumentation amplifier launched by the American ADI company. It has the characteristics of high accuracy, low offset voltage (maximum 50uV), and low offset drift (maximum 0.6uV℃/), and its maximum operating current is only 1.3mA, only an external resistor is needed to set the gain, and the gain range is 1 to 10000. In addition, AD620 adopts 8-pin SOIC and DIP package, the size is smaller than the discrete circuit design, and the power consumption is lower, so it is suitable for battery-powered instrument applications.

Figure 3 AD620

Figure 3 AD620

 

Because its input stage uses Superβeta processing, it can achieve a low input bias current of up to 1.0 nA. AD620 has a low input voltage noise of 9 nV/√Hz at 1 kHz, a peak-to-peak noise of 0.28μV in the frequency band from 0.1 Hz to 10 Hz, and an input current noise of 0.1 pA/√Hz, so it can be used as a preamplifier well. At the same time, the 0.01% settling time of AD620 is 15μs, which is very suitable for multiplexing applications; and the cost is very low, enough to realize the design of an instrumentation amplifier per channel.

 

2.3 Microcontroller PIC10F206

 

PIC10F206 is a low-cost, 6-pin 8-bit flash microcontroller introduced by the US Microchip company that uses RISC architecture. PIC10F206 has 512 words of FLASH, 24 bytes of SRAM, watchdog timer (WDT), power-on reset circuit (POR) and device reset timer (DRT) and 4MHz internal oscillator, thus eliminating the need for external reset circuit and the crystal oscillator, reduces system cost and power consumption, and enhances system reliability. It also has a wide operating voltage range (2·0V to 5.5V). The above characteristics make it suitable for applications in price-sensitive and battery-powered areas.

Figure 4 PIC10F206

Figure 4 PIC10F206

 

2.4 Amplifying Circuit of Galvanometer

 

The amplifying circuit of the galvanometer takes the instrument amplifier AD620 as the core element, as shown in Figure 5.

 

The differential signal output by the DC bridge is input from the socket J1, passes through the anti-radio frequency interference low-pass filter circuit formed by R1, R2, C1, C2, C3, and reaches the instrumentation amplifier AD620 after weakening the interference signal. D1, D2 and R1, R2 together form the input protection circuit of the galvanometer, which can withstand input voltages of tens of volts. R3, R4 provide a loop for the input bias current of AD620 [1] to ensure that it can work stably and reliably.

 

The resistance R0 and potentiometer RP1 between pin 1 and pin 8 of AD620 are gain adjustment resistors, denoted as RG. R0 is connected in series with RP1 to limit the upper limit of the magnification to 495 times. The potentiometer RP2 and R5, R6 form the zero adjustment circuit of the galvanometer together, realize zero adjustment by changing the voltage of the REF pin of AD620. R7 and C7 form the output low-pass filter of AD620. R7, D3, D4 constitute the protection circuit of the pointer meter head.

Figure 5 The amplifying circuit of galvanometer

Figure 5 The amplifying circuit of galvanometer

 

The maximum sensitivity of a galvanometer is usually 10uV/degree to 15uV/degree, which can well meet the requirements of the experiment. The internal resistance Rg of the galvanometer meter head is 4.7kΩ, the gain adjustment resistance RG=R0+RP1, take R0=100Ω, R7=1kΩ, when RP1 is 0Ω, the amplification factor of AD620 is

Taking into account the partial pressure of R7 and the internal resistance Rg of the meter, the magnification of the galvanometer is

The current sensitivity of the meter head is 1μA minute/degree, so the voltage sensitivity of the meter head is 4700μV minute/degree. The sensitivity of the galvanometer is

 

S=4700/G′=11·5μV/division

 

Meet the requirements of physical experiments.

 

2.5 Power Monitoring and Power Management Circuit of Galvanometer

 

Figure 6 is the power monitoring and power management circuit of the galvanometer. The GP2 pin of the microcontroller PIC10F206 is connected with the P-channel MOSFET tube Q1, the purpose is to control whether to supply power to the amplifying circuit of the galvanometer. When GP2 output is low level, Q1 is turned on, and the system supplies power to the amplifier circuit. The role of ICL7660 in power conversion here is to convert +VS to -VS.

 

R8, R9, and the analog comparator inside PIC10F206 together form the power supply voltage monitoring circuit. The negative input terminal CIN of the analog comparator is connected to the internal reference voltage of the single-chip microcomputer  (the nominal value of the voltage is 0·6V), and the positive input terminal CIN+ is connected with the partial pressure of R8 and R9. Because the lowest operating voltage of AD620 is ±2·3V, from a conservative point of view, the lower limit of the operating voltage is set to ±2.5V. When the positive power supply voltage is less than 2.5V, the voltage of CIN+ is less than 0.6V, and the comparator output is reversed. After the microcontroller detects this event, the GP2 outputs a high level turns off the power supply of the amplifier circuit, and then executes the SLEEP instruction to make the microcontroller enter sleep status.

 

Figure 6 Power monitoring and power management circuit of galvanometer

Figure 6 Power monitoring and power management circuit of galvanometer

 

In order to save power consumption, when the working voltage is normal, the galvanometer should enter the sleep state (ie soft shutdown) after working for a period of time, and it can be awakened at any time when it needs to work. Button S1 is set for this function. The working time of the galvanometer is preset to 15 minutes. When the working time is up, the GP2 of the single-chip microcomputer outputs a high level, shuts off the power supply of the amplifying circuit, and then executes the SLEEP instruction to make the microcontroller enters the sleep state.

 

In the sleep state, if S1 is pressed, the pin level of GP3 will change. This event will reset the microcontroller (note: PIC10F206 has no conventional interrupt function) and wake up the microcontroller. In the sleep state, the current consumption of the galvanometer is less than 0.1μA, which is very power-saving, so there is no need to worry about forgetting to turn off the power supply of the galvanometer and consuming the battery.

 

III. Software System

 

When the galvanometer is powered on, the microcontroller first performs system initialization, then supplies power to ICL7660 and AD620, and then turns on the analog comparator to check whether the supply voltage is appropriate. If the voltage is appropriate, continue to supply power to the amplifier circuit, and then execute a 15-minute countdown. If S1 is pressed during this period, the timing time is reset to 15 minutes. When the 15-minute countdown is up, the single-chip microcomputer turns off the power supply of the amplifier circuit, and then enters the sleep state (ie, soft shutdown). The flow chart of the procedure is shown in Fig. 7.

Figure 7 Flow chart of the procedure

Figure 7 Flow chart of the procedure

 

IV. Conclusion

This article discusses the galvanometer based on instrumentation amplifier AD620 and microcontroller PIC10F206, which has stable performance and low power consumption. The current is about 4mA during normal operation, and the current consumed in sleep mode is less than 0.1uA, which is very suitable for battery power supply. The use of two AA batteries for power supply saves battery cost. The soft shutdown function can effectively prevent battery consumption caused by forgetting to turn off the power.


FAQ

  • What is AD620?

AD620 is a low-cost, high-precision instrumentation amplifier. It only requires an external resistor to set the gain. The gain range is 1 to 10,000.

  • Can I change AD620 to AD623 when making MCU products?

Both AD620 and AD623 are single instrumentation amplifiers, and the pin arrangement is exactly the same.

The main difference is: AD620 must use positive and negative power supplies, AD623 can be a positive and negative power supply or a single power supply.

If the original board is AD620, you can replace it with 623; if the original board is AD623, you may not be able to replace it with 620 (it depends on whether the power supply of the original board circuit is dual power supply or single power supply).

After replacing AD620 and AD623 in single-chip products, the program can work normally without modification.

  • What is the difference between AD620BR and AD620AN?

Their packages are different.

  • What is the output resistance of AD620? How to adjust it?

AD620 is a kind of low power consumption instrument amplifier, its output resistance is about 10K, this is the inherent characteristic of this chip, generally it is difficult to adjust.

If you have requirements for output resistance, you can generally use an external circuit to solve it.

  • Is AD620 a positive phase amplification or a reverse phase amplification?

AD620 is an instrument amplifier, the output voltage is [(Vin+)-(Vin-)]*gain.

If the desired signal is (Vin+)-(Vin-), the gain is positive, which is equivalent to positive amplification.

Conversely, if the desired signal is (Vin-)-(Vin+), the gain is equivalent to negative, which is equivalent to reverse amplification.

  • What is an instrumentation amplifier?

Instrumentation amplifier, an improvement of the differential amplifier, has an input buffer, does not require input impedance matching, so that the amplifier is suitable for measurement and electronic instruments

 

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