High accuracy based on AD7714 isolated data acquisition system (1)

　　

Summary overview of 24-bit Σ-△ a/d converter performance and features AD7714, explaining the chip in a high-precision measuring instruments.

Describes how to use AD7714 multi-channel, multiple range DC voltage measurement, highlighting SPI data bus photoelectric isolation implementation approach and practice under engineering improve the antijam capability of the production of printed circuit boards. Use the above approach, the data acquisition system successfully combines multiple AD7714, multi-channel μ A level of precision measurement. The article gives the relevant circuit schematics and MCS and AD7714 interface instance.

Keywords AD7714 photoelectric isolation SPI data acquisition

In high precision and multi-channel sampling devices, A/D chip selection of appropriate or not on the overall performance of the system is key to performance is good or bad.

Currently, because the digital signal processing technology for the rapid development of data acquisition device to the channel also constantly improving, particularly on devices sampling resolution, sampling rate and sampling channel number, and other parameters of the requirements become more stringent.

This system of measurement using polarization relay follows control box instrument design, measuring data ranges, measurement accuracy is high, the number of channels.

At the same time, since the system measurement circuit relatively complex, the signal interference between easy, high-speed operation of the electrical signal interference will cause system crashes. In response to the above, the author adopts multiple-input, high-precision A/D converter AD7714, and communications between the MCU adopts photoelectric isolation technology.

1 the basic situation of AD7714

AD7714 is a complete for low-frequency measurement of analog front-end applications.

It's 3-wire serial interface with SPI, QSPI, MICROWEIR compatible. The software can gain setting, signal polarity and channel selection to configure. AD7714 's main characteristics are as follows:

◇ The highest achievable accuracy code output 24-bit, while ensuring the nonlinearity of 0.0015 per cent;

◇With front-end gain programmable amplifiers, gain values are 1 ~ 128, with programmable low pass filter and can read and write system calibration coefficient;

◇ 5 channel input, according to the need for a 3-channel differential input or 5 Road, quasi differential input;

◇ Low noise (rms < 150 nV);

◇ Low typical current value is 226 μ A (power saving mode only 4 μ A);

◇ Single 5 V power supply (AD7714-5) or a single 3 V power supply (AD7714-3).

AD7714 provides 24 feet DIP, SOIC, SSOP-28 pin TSSOP Package.

The PIN function as shown in Figure 1 (for example by 24DIP package). AD7714 function block diagram shown in Figure 2.

　　

　　

　　

　　

2 the practical application of the system

2.1 AD7714 peripheral interface configuration

POL clock polarity.

Input low level, the data transfer operation serial clock first jump from low level to high level. Input high level, the data transfer operation serial clock first jump from high to low level.

RESET logic input.

Low level valid input, the device of control logic, interface, logic, digital filters, as well as analog modulator reset to power-up state. This system is in DIN input writes a series of 1 for software reset to return to wait on AD7714 communication registers for write operations.

Used software reset to note two things: first, the DIN line write AD7714 logic 1 at least up to 32 serial clock cycle; the second is written to any register information is unknown, and thus to set all the register again.

CS chip select.

To select a valid AD7714 low level logic input. When this input by hardware line is set to a low level, the work in its AD7714 3 line interface mode.

　　

SYNCL logic input.

When using multiple AD7714, it is used for digital filters and synchronization of analog modulator.

2.2 AD7714 and MCU interface

AD7714 and MCU interface between the relationship shown in Figure 3, Figure 3 shows the input and output circuits to convert.

In order to be able to obtain stable data, between the MCU AD7714 joined optical isolator. Optical isolation device using isolation voltage, high speed, high common-mode rejection of 6N137. Because of the speed, 6N137 programming does not require an appropriate delay can meet the optical isolator level creation time. Testing indicates that through optical isolator, gain stability data can increase 3 ~ 4 bits (binary digits).

AD7714 CS grounding to work on AD7714 always SPI interface mode.

For every interface module control is three-State through 74LSl25 allows client to achieve. On one of the SPI interface operation, so that its State is gated 74LSl25, while other SPI interface 74LSl25 is high impedance state. This enables an individual to the microprocessor interface operation, without affecting other interfaces. CPU through the 3-State buffer 74LSl25 control, you can implement multiple interfaces share the same cable. Figure 3 DA-CS is control with SPI bus D/A converter IC.

　　

6N137 external components resistance under

Own actual situation to select that should not be too large or too small. In order to increase stability, preferably in the resistance at the edges and on the electrolytic capacitors.

　　

　　

AD780 2.3 voltage reference

High-accuracy reference voltage source and provide the basis for AD7714 AD780.

Its basic characteristics: by 8 feet of vacant or grounding, achievable 2.5 V or 3.0 output; V output voltage range in 2.5 V1 mV or 3.0 vl mV; input voltage range from 4 to 36 V to achieve 2.5 V or 3.0V output.

Made using the AD780 reference voltage source, its surrounding capacitor must follow Figure 4 on the configure, otherwise, the output of accuracy will decrease considerably.

Special attention should be paid to the output port and AD780 100 μ F capacitor, test shows that the capacitor can make system accuracy of 2 ~ 3 bits (binary digits).

　　

　　

2.4 AD7714 analog front end

In the analog front end to measurement of current and voltage are many and vary greatly.

In order to be able to improve measurement accuracy, must, in accordance with the voltage and current size and design sensible voltmeter and ammeter. In Figure 5, through reasonable to select the resistance of R1 and R2, and make it short or open circuit, to achieve the voltmeter and ammeter.

　　

Design ammeter, short-circuit the R1, select the appropriate R2 resistors, resistor R2 is through AD7714 maximum input voltage, and the measurement of maximum current.

Design voltage meter, if measurement voltage in AD7714 input voltage range, is short circuit, open circuit Rl R2; if the direct measurement of the voltage measuring over the range of input voltage AD7714, reasonable select R1 and R2 resistance pressure, to meet the requirements of the measurement.

AD7714 transport people channel by AIN (+) and AIN (a) are arranged in pairs, AIN (+) terminal on the unipolar and bipolar signals as a reference voltage is the respective AIN (1) input voltage on the client.

For example, if you switch SW in Figure 5 position, i.e. the AIN (+) access AD780 output voltage + 2.5 V, AD7714 configured as single polarity, to set the gain of 2, then it AIN (+) input voltage range of + 2.5 ~ + 3.75 V. If in the same configuration, to bipolar, AIN (+) input voltage range of + 1.25 ~ + 3.75 V (i.e. 2.5 Shi 1.25 V). If you make it through SW switching AIN (a) to AGND, devices not configured for more than 30 mV bipolar ranges.

　　

　　

If the external voltages and currents are very interference, accuracy of measurement will be greatly affected.

In each AD7714 analog input terminals are coupled with a capacitance (as in Figure 5, C1 and C2), through the experiments show on its accuracy has greatly improved. Capacitor selection to your conversion rate, and select external interference.

2.5 LDC5-24S5 power module

External can provide power is 27 V, poor stability due to the power supply as well as from external interference can affect the system's normal work, while at the same time the author of the power required to operate at a single + 5 V.

Therefore, the use of a DC LDC5-24S5 DC switching power supply modules, input voltage range 18-36 V DC voltage, output is a single + 5 V. The power module has a very good role of DC-DC isolation, the output voltage is very stable, it can be used directly as the power supply, thus AD7714 simplifies the entire circuit.

3 practical problems and solutions

How to filter out the analog input 3.1-interference

AD7714 analog input terminal can accept unipolar or bipolar input voltage range.

Bipolar input range does not mean that the device in analog input terminal handles negative voltage. In order to ensure that the device is working, the analog input that cannot change to a more than 30 mV. This system requires a test voltage is 27 V, it consists of a high speed motor device output. If not handled directly tested by the motor is too low or too high negative pulse is the pulse will affect AD7714 normal work, serious permanent damage when AD7714 chip. The author by a lot of analysis and testing, drew as shown in Figure 6 the ideal filtering pulse circuit. Which LI and I.2 is double hole core winding, C3 's size according to own actual situation to measurement.

　　

　　

3.2 how to make the printed circuit board and component Assembly received

In order to get the best performance of the ADC, you must use the analog and digital separate printed circuit boards.

In the printed circuit board design, in particular, [1] [2]