Application of AD7794 in high-precision and low-power measurement device

1 Introduction

In modern electronic measurement, there are increasingly higher requirements for measurement accuracy. At the same time, due to the battery power supply in the field, there are also high requirements for the power consumption of the overall circuit. For example, in differential pressure flow measurement/metering, the signal given by the pressure sensor is very weak, which puts high demands on the DC amplifier and ADC circuit. Traditional precision data conversion and system stability solutions cannot combine low noise, low drift and low power consumption characteristics, and often have to sacrifice certain performance. In response to this special and universal demand in the field of industrial measurement, AD7794 adopts a composite structure that combines chopper amplifier circuit (suppressing drift), AADC (improving accuracy and suppressing noise) and low power consumption to form a relatively ideal special device with the above excellent performance. At the same time, the device is very small, which is convenient for use in various equipment.

Based on the author's experience in the actual design of inner cone intelligent industrial gas meters, this paper summarizes the characteristics of the high-precision A/D conversion chip AD7794 and describes its use.

2 AD7794 Functions and Technical Characteristics

AD7794 provides almost all functions required by instrumentation applications, thus reducing the design workload and saving many peripheral devices. AD7794 has low power consumption and full analog input terminals, which can be used in the measurement of low-frequency signals. It overcomes the limitations of noise and power consumption in similar products and can provide low noise and low power consumption characteristics at the same time. This series of ADCs is powered by a single power supply of 2.7v~5.25v, and its full power consumption current is only 400μA, while the noise is only 40nVrms, making it suitable for applications requiring low power consumption and high-precision measurement. It integrates a 24-bit ADC with six differential sensing channels, making it very suitable for applications requiring more channels. These six differential channels can be combined into differential signals and differential reference inputs in pairs, which can effectively overcome common-mode interference. There is also a low-noise, low-temperature drift gain stage instrument amplifier circuit on the chip, and the gain can be set as needed. In addition, the chip also integrates an adjustable gain excitation current source and a bias voltage generator for temperature measurement. The chip can use an internal clock, and an external clock can also be used if multiple chips are run synchronously. The sampling rate, which is also the rate at which data is output, can be programmed between 4 Hz and 500 Hz. At certain rates, such as 16.6 Ih, it can provide the function of simultaneously suppressing 50 Hz and 60 Hz interference signals.

Figure 1 shows the simplified structure of AD7794, which is a ∑-△ modulated analog-to-digital converter suitable for narrowband and high-resolution applications. The ∑-△ modulator of AD7794 converts the sampled input signal into a digital pulse train, whose "1" density includes digital information. After digital filtering and extraction, high-resolution low-rate data is output. The ∑-△ modulator also has the effect of noise reduction, because the high sampling rate lowers the noise floor, and most of the noise (high-end spectrum part) is filtered out after filtering. The higher the order of the modulator, the more obvious the effect of noise suppression within the useful bandwidth. However, higher-order modulators are prone to instability. Therefore, a trade-off must be made between the order of the regulator and stability. In narrow-band ∑-△ analog-to-digital converters, second-order or third-order regulators are usually used, so that the device has good stability.

Figure 1 AD7794 internal simplified structure module diagram

The low noise instrumentation amplifier of the AD7794 can be operated in chopper mode. The chopper is an embedded component of the AD7794 that can be used to eliminate errors caused by drift. The working principle of the chopper is to alternately invert (or clip) the output of the multiplexer, the input component of the analog-to-digital converter. Then, the positive and negative signal segments of each chop are converted to analog-to-digital once. Next, the digital filter is used to average the results of these two conversions. In this way, any offset errors that occur in the analog-to-digital converter are eliminated, and more importantly, the impact of temperature on offset drift is minimized.

3 Application Circuit Design of AD7794

Figure 2 shows the application block diagram of AD7794. AD7794 has a simplified synchronous serial interface, which is easy to connect to the microcontroller MC. The serial interface, ADC, chopper instrument amplifier and multi-channel structure in AD7794 form a full ADC type - instrumentation ADC.

Figure 2 Typical application of one channel of AD7794

Among them, MSP430F1611 is an ultra-low power mixed signal processor with one active mode (AM) and five low power modes (LPM0~LPM4). In standby mode, its power consumption is 0.7uA; in power saving mode, it can be as low as 0.1uA. The connection between AD7794 and MSP430F1611 is very flexible. The following describes the design of a typical sensor and conditioning circuit, as shown in Figure 3. Among them, AD7794 has three sets (reference voltage and measured voltage) and six differential input terminals, and the circuit can be connected to any one set.

Figure 3 AD7794 input circuit design

The whole circuit is mainly composed of sensor bridge and signal conditioning circuit. The sensor inputs the signal in differential mode, that is, it is represented by the voltage difference between the positive and negative ends of the output. When the measured non-electrical changes, the resistance value of the sensor will change, and this change will be linearly reflected in the potential difference (voltage) at the left end of R7 and R9. By collecting the difference signal of this potential, the measured value and its change can be calculated. The analog sensor signal is sent to AD7794 through the differential ports AD7794-AIN+ and AD7794-AIN- for digital-to-analog conversion. In actual use, it is possible that the input analog signal voltage is disturbed and has a large range of fluctuations. If the signal on the sensor is directly connected to AD, in extreme cases, such as transient electrostatic high voltage, it may cause permanent damage to AD7794. Therefore, diodes D1, D2, D3 and D4 are used in the circuit to clamp the input signal within a safe range, thereby playing a role in overvoltage (including positive and negative) protection. Resistors R7, R8, R9 and R10 are used as current limiting resistors (their resistance has almost no effect on the signal), further protecting the subsequent circuits. Cl and C2 can effectively filter out the radio frequency interference entering the circuit, which is particularly effective for areas close to radio stations.

The reference voltage of AD7794 can be taken from the inside or outside. However, when measuring external bridge signals, it is more effective to use an external reference voltage, so an external reference is used in this circuit. When using AD7794 to measure tiny signals, the on-chip low-noise instrumentation amplifier will be used, which can effectively reduce the interference of external noise. For example, when the gain of the internal amplifier is 64, the typical effective value of the introduced noise is only 40nV. However, when the gain of the op amp is greater than or equal to 4, its common-mode voltage cannot be too low, otherwise it will deteriorate the characteristics of the op amp. As required, when the AD7794 works in chopping mode, the input common-mode voltage ((AD7794) + (-)) must be greater than. , so that the dynamic range of the input signal can be guaranteed; and, when using the internal amplifier, if the external reference voltage VREF used is close to the analog power supply AVDD, the actual input analog signal value cannot exceed 90% of (Vr.lgain), otherwise the linearity of the AD at both ends of the input signal will deteriorate. In order to solve this problem well, R6 and R12 are used in this circuit, so that the reference voltage AD7794 REF+ and AD7794 REF- of AD7794 will not be close to the limit voltage of the analog power supply. The whole circuit adopts bridge input, so that when the external power supply fluctuates in a small range, it can ensure that the actual differential voltage added to the amplifier and the input reference voltage are not affected by the outside world.

AD7794 uses offset binary coding. When a unipolar signal is used (Ain+ - Ain > 0), the relationship between its input voltage and output data is:

This D directly represents the measured value. Here, G is the total gain. (REF+ - REF-) is the differential reference voltage, and (Ain+ - Ain-) is the input differential voltage signal.

When a bipolar signal is used (REF>Ain+ - Ain>0 or 0>Ain+ - Ain>-REF), the output characteristic becomes:

When an external reference voltage is used, since R5, R7, and R11 can be ignored, we have:

In an ideal sensor,

R is the total resistance of the measurement bridge, that is, the resistance of the bridge arm. In static state, R1=R2=R3=R4=R is the resistance change of each arm during measurement. It can be seen that when using an external reference voltage (also used as a sensor voltage excitation), the ADC output data is directly related to the sensor change, that is, the measured value, and has nothing to do with the actual value of the reference voltage! This greatly reduces the stability requirements of the reference voltage. Of course, the reference voltage must meet the value requirements of the ADC, and it is still required to remain unchanged (short-term stability is sufficient) during a measurement (conversion).

The whole system is suitable for occasions requiring high precision and low power consumption. Here, the voltage that provides excitation to the entire bridge, that is, the reference voltage, is taken from the DAC output of the MSP430F1611. In this way, the accuracy and stability of the applied voltage can be guaranteed, and the power supply to the bridge can be turned off at any time when the measurement signal is not needed. When the MSP430 microprocessor enters a dormant state, the chip of the entire system and the circuit can also be put into a dormant state, which can further reduce the power consumption of the system.

4 Experimental and measured results of AD7794

In order to simulate the actual use of AD7794, R1, R2, R3, and R4 in Figure 3 are all replaced by a resistance box, and the accuracy and resolution of the resistance box is 0.1 ohm. In the actual test, the resistance change used is 1 ohm. During the test, R1 and R4 are used as a group, and R2 and R3 are used as a group. The two groups of resistance values ​​change in different directions, that is, one group is adjusted up and the other group is adjusted down, so as to simulate the pressure difference change on the sensor, and then the measured data is plotted into a curve. The experimental results are shown in the figure below. Among them, the vertical axis is the measured AD conversion result, and the horizontal axis is the pressure difference change rate, that is, (V+-Vv)/(VRF+-VREF_).

As can be seen from Figure 4, the performance of the entire AD7794 is satisfactory. The entire AD conversion result changes linearly with the actual input signal, and the measured maximum linear error is less than 1‰.

5 Conclusion

The system has been successfully applied to low-power gas metering devices and operates stably and reliably. Compared with traditional solutions, this system has high accuracy, low power consumption, strong anti-interference ability, easy debugging, small size, and is suitable for various applications such as handheld, outdoor, and solar power supply. This method can be promoted and applied to a wide range of industrial measurement applications for low-frequency, slow-changing signals.

The author's innovation point is: proposed the design principle and method of high-precision ultra-low power measurement and metering device based on AD7794, and gave a specific hardware circuit, which provides a certain reference for the further development of other types of measurement devices.