Features

Low power: 330 Weian's maximum static current rail to low noise and low inconsistency; 8 nv/√Hz 1 kHz, the maximum input voltage noise; 0.15 μV P-P RTI noise (G 100); 0.5 PPM non -linear, 2 kΩ load (g 1); excellent communication specifications; 10 kHz minimum co -mode inhibition ratio is 80 dB (g 1); DC performance ( AD8422 BRZ); minimum co -mode inhibitory ratio 150 decibels (g 1000); maximum gain error 0.04%(g 1000); 0.3 micro volt/degree Celsius maximum input offset drift; 0.5 na na The maximum input bias current; wide supply range; 4.6 V to 36 V single power supply; ± 2.3 V to ± 18 V dual power supply; input overvoltage protection: 40 V from the opposite power supply; gain range: 1 to 1000.

Application

Medical equipment; industrial process control; response meter; sensor interface; precision data acquisition system; channel isolation system; portable instrument.

General description

AD8422 is a high -precision, low power consumption, low noise, rail -paired instrument amplifier, which provides the best performance of the industry units Weire. AD8422 processing signal with ultra -low distortion performance, which has nothing to do with the load throughout its entire output range.

AD8422 is the third -generation product of the AD620 industrial standard. AD8422 uses new process technology and design technology, which has a higher dynamic range and lower errors than previous products, while the power consumption is less than one -third. AD8422 uses high -performance pins introduced by AD8221.

The extremely low bias current makes the AD8422 no error and has high source impedance, allowing multiple sensors to be reused to the input terminal. Low -voltage noise and low -current noise make AD8422 an ideal choice for measuring Whist Tong Bridge.

The wide input range and rail output of AD8422 brings all the benefits of AMP to the high performance of AMP to a single power supply. Regardless of the use of high or low power voltage, the saved power makes AD8422 a very good choice for the number of Gaoxin Road or the power sensitive application of power, in a very nervous wrong budget.

AD8422 uses a stable input protection to ensure reliability without sacrificing noise performance. The AD8422 has a high ESD resistance, and the input terminal is protected by continuous voltage of up to 40 V from the opposite power rail.

A resistor sets the gain to 1 to 1000. The reference pin can be used to apply accurate offset to the output voltage.

The temperature range of AD8422 is -40 ° C to+85 ° C, and its typical performance curve is 125 ° C. In8 -guide MSOP and 8 -guide SOIC packaging.

Typical performance features

t 25 ° C, v ± 15, v 0 v, r 10 kΩ, unless otherwise explained.

] Operation theory Architecture

AD8422 Based on the classic 3rd throughput amplifier topology. This topology structure is divided into two stages: the front amplifier that provides differential amplification and the differential amplifier that eliminates the co -mode voltage. Figure 55 shows the simplified schematic diagram of AD8422.

On topology, Q1, A1, R1 and Q2, A2, R2 can be regarded as accurate current feedback amplifiers, maintaining a fixed current in the launch pole of Q1 and Q2. Any change in the input signal will force the output voltage of A1 and A2 to change accordingly, and keep the Q1 and Q2 current on the correct value. This will cause the diode to precisely drop from –In and+in to node 3 and node 4, so that the differential signal applied to the input is copied by the R pins. Any current of RG must also flow through R1 and R2, so as to generate differential voltage obtained between nodes 1 and node 2.

The magnified differential signal and co -mode signal were applied to the differential amplifier.

Laser fine -tuning resistance allows high -precision amplifiers with a gain error of less than 0.01%, and the co -model suppression ratio of more than 94 decibels (G 1). The power current is accurately fine -tuned to reduce the uncertainty caused by local changes in power loss and noise. High -performance scooter and special concern for design and layout allows high -coefficient suppression ratio within the broadband and temperature range. AD8422 uses super β input transistor and bias current compensation, providing extremely high input impedance and low bias current, and extremely low voltage noise. At the same time, only 300 Weian power current is used. The overvoltage protection solution allows input to pass 40V from the opposite track without affecting noise performance.

The transmission function of AD8422 is:

123] Place a resistor on the RG terminal to set the gain of AD8422. This gain can be calculated by referring to Table 6 or using the following gain equation:

When the gain is not used, when the gain is not used When a resistor, AD8422 defaults to g 1. Add the tolerance and gain drift of the RG resistor to the specifications of AD8422 to determine the total gain accuracy of the system. When the gain resistance is not used, the gain error and the smallest drift.

RG power consumption

AD8422 to copy the differential voltage at the input terminal of the RG resistor. Choose an RG resistance size to process the expected power consumption at the ambient temperature.

Reference terminal

The output voltage of AD8422 is generated according to the potential on the reference terminal. This can be used to accurately offset the output signal application. For example, the voltage source can be binded to the REF tube foot to make the output level offset, which allows the AD8422 driver single polar modulus converter (ADC). The REF pin is protected by the ESD diode and must not exceed+V or -V more than 0.3 V.

In order to obtain the best performance, the source impedance with the REF terminal is lower than 1Ω. As shown in Figure 55, the reference terminal Ref is at the end of the 10 kΩ resistor. The additional impedance on the REF terminal increases the 10 kΩ resistor and causes signal amplification connected to the positive input terminal.

The magnification of the additional R can be calculated as:

Only the positive signal path is amplified; the negative signal path is not affected. This uneven magnification reduces the co -mode suppression ratio.

The input voltage range

The 3 operation amplifier structure of AD8422 At the first level of the first level of the agglomerator level at the differential amplifier level. Internal nodes between the first and second levels (nodes 1 and node 2 in Figure 55) experienced the combination of signal, co -mode signal and diode drop. Even if a single input and output signal are not limited, the voltage source can limit the combined signal. Figure 10 to FIG. 13 shows this limit in detail.

Layout

In order to ensure the best performance of AD8422 at the printing circuit board level, please pay attention to the design of the circuit board layout. To help complete this task, the pin of AD8422 is arranged logically.

Common model suppression ratio

Poor layout will cause some co-mode signals to convert to differential signals before reaching IN-AMP. This conversion occurs when one input path has a frequency response that is different from another. In order to maintain a high co -mode suppression ratio, the input source impedance and the capacitance of each path should be closely matched. In the input path (for example, for input protection) near the input amplifier input, additional source resistance will be placed, which will minimize their interaction with parasitic capacitors from PCB records.

Parasitic capacitors at the gain setting pin (R) will also affect the frequency of co -mode suppression ratio. If electricityThere is a component (for example, a switch or jump line) at the pins of the gain setting, and select a component that makes the parasitic capacitor as small as possible.

Power and ground

Use a stable DC voltage to power the instrument amplifier. The noise on the power insertion of the power will adversely affect the performance.

The 0.1 μF capacitor is closer to each power. Because the length of the barrier container is very important at high frequency, it is recommended to use the surface sticker capacitor. Parasitic inductance in the barrier -connected ground line plays a low impedance of bypass capacitors. As shown in Figure 58, 10 μF capacitors can be used at places far away from the device. For a larger value capacitor at a lower frequency, the distance between the current back path is not important. In most cases, such capacitors can be shared by other local precision into circuits.

The ground floor layer helps reduce the parasitic inductance. This makes the voltage decreased as the current changes. The area of the current path is directly proportional to the size of the parasitic inductance, so it is directly proportional to the impedance of the high -frequency path. The major changes in the current in the inductance go coupling path or the current circuit will have unnecessary effects because these changes are coupled into the amplifier input.

Because the load current flows out of the power supply, the load is connected to the same physical location as the bypass capacitor.

Reference foot

The output voltage of AD8422 is generated according to the potential on the reference terminal. Make sure that REF is connected with appropriate local grounding.

Input bias current return path

The input bias current of AD8422 must have a DC circuit ground. When using a floating source (such as armocouple) with no current back path, a current return path is created, as shown in Figure 59.

The input voltage exceeds the power rail

Many instrument amplifiers specify a good co -mode suppression ratio and input impedance, but in the actual system, due to the input protection due to input protection The required external components are affected. AD8422 has a very powerful input. It usually does not require additional input protection. Without damaging parts, the voltage input from the opposite power rail can be as high as 40 V. For example, using +5 V positive electrode power and 0 V negative electrode power supply can safely bear the voltage of -35 V to +40 V. Unlike other meter amplifiers, components can process larger differential input voltages, even if the part is at high gain.

For the input voltage of relative orbit less than 40 V, no input protection is required.

Keep the remaining AD8422 terminals in the power supply. All terminals of AD8422 have anti -static protection.

The input voltage exceeds the maximum rated value

For applications that encountered AD8422 with a voltage limited by absolute maximum rated value, external protection requires external protection. This external protection depends on the duration of the overvoltage and the required noise performance.

For short life events, only transient protectors such as metal oxide resistors (MOV) may be needed.

For longer events, the input -connected resistor with a diode combination is used. In order to avoid deterioration of bias current, low leakage diode, such as BAV199 or FJH1100s. The diode prevents the voltage of the amplifier input exceed the maximum rated value, and the resistor limits the current of the diode. Because most external diode can easily handle 100 mAh or more, the resistance value is not large. Therefore, the effect of protecting resistance on noise performance is minimal.

Another solution is to use series resistors to sacrifice some noise performance. In the case of overvoltage, the current of input AD8422 is limited to the safety value of the amplifier internally. Although the AD8422 input must still be maintained within the absolute maximum rated range, the maximum voltage that the system can withstand will be increased to the following value by protecting the I × R drop by protecting the resistor:

For positive input signals, [123 123 ]

For negative input signals,

Overvoltage performance shown Essence When the gain is greater than 100 and the power supply voltage is less than ± 2.5 V, the excess driving voltage beyond the track may cause the output to reverse to the reference pin voltage.

RF interference

When the amplifier is used in the application with a strong RF signal, RF school is usually a problem. Interference can show a small DC offset voltage. The high -frequency signal can be filtered by the low -pass RC network placed at the input end of the instrument amplifier, as shown in Figure 62.

The filter is limited to the input signal bandwidth according to the following relationship:

Among them

CD affects the differential signal, CC affects the common mode signal. Select R and CC values that minimize RFI. The non -matching between the R × CC and the R × CC of the negative input reduced the CMRR of AD8422. By using a magnitude larger than C C, it reduces the effect of loss and improves performance.

The resistance will increase noise; therefore, the selection of resistors and capacitors depends on the balance required between noise, high -frequency input impedance and radio frequency resistance. The resistor used by the RFI filter can be the same as the resistor for input protection.

Application information

Precision bridge adjustment

A high -co -model suppression ratio, low drift and rail -to -orbit AD8422 is the best choice to regulate the Whist Tong bridge signal. Using the appropriate power supply voltage, you can adjust the gain and reference foot voltage, so that the output of the full margin bridge is matched with the output range of any expectation, such as 0 V to 5 V. Figure 63 shows the use of AD8276 low power, precision differences, and ADA4096-2 low power to convert the bridge signal to circuit with 4mo to 20 mAh output, rail-to-orbit input and output, and over-voltage protection op amp. For high -precision bridge circuits, pay attention to compensation offset and temperature errors. For example, if the voltage at the reference pin is used to compensate the bridge offset, make sure that AD8422 is within the range of its maximum expected offset. If you do not include zero potential meter, the input of the positive operation amplifier is connected to the center of the 24.9 kΩ and 10.7 kΩ division, and the voltage of the divisioner is 1.5 V. If AD8276 and ADA4096-2, make sure the expected output voltage of AD8276 is within its output range, V is within the input and output range of ADA4096-2. The transistor must have enough breakdown and I. Low -cost transistors, such as BC847 or 2N5210, are recommended.

Process control analog input

In the process control system, such as programmable logic controller (PLC) and distributed control system (DCS), the simulation variables usually only appear only in several standard voltage voltage Or within the range, including 4-20 mAh and ± 10 volts. The variables in these input range must be obtained or attenuated frequently, and the level offset is to match the specific ADC input range, such as 0 V to 5 V. The circuit in FIG. 64 shows a method that can be implemented by a single AD8422.

Low -power, overvoltage protection and high precision make AD8422 very suitable for process control applications. High input impedance, low bias current and low current noise allow significant source resistance and minimum additional errors.

The size of the shape