In practical applications, the increasing RF interference (RFI) must be handled, in particular the case where the signal transmission line is longer and the signal intensity is low, and the typical application of the meter amplifier is this case because its intrinsic common mode suppression Ability, it can extract weak differential signals from stronger common mode noise and interference. However, there is a potential problem that is often ignored, namely the radio rectification problem existing in the instrument amplifier. When there is a strong radical interference, the integrated circuit may rectify the interference, and then exhibit in DC output offset error. The common mode signal of the instrument amplifier input is usually attenuated by its common mode suppression. RF rectification still occurs, because even when the best instrumentation amplifier is actually inhibiting common mode noise when the signal frequency is higher than 20 kHz. The input stage of the amplifier may rectify the strong RMS and then exhibited in a straight-out loss error. Once rectified, the low-pass filter in the instrument amplifier output will not be able to eliminate this error. If RF interference is intermittent, it will result in a measurement error that cannot be perceived.
Design practical RF interference filter
The most practical solution to this problem is to use a differential low pass filter prior to the meter amplifier to attenuate the RF signal. This filter has three effects: to eliminate the radio frequency energy in the input line as much as possible; keep the AC signal between each line and the ground (shared) to balance; and maintain a sufficiently high input impedance over the entire measurement bandwidth, Avoid adding the load of the source.
Figure 1 is a basic block diagram of a variety of differential radio frequency interference filters. The element values shown in the figure are selected for the AD8221, and the -3db typical bandwidth value of the AD8221 is:
Figure 1 Low pass filter circuit for preventing radio frequency interference rectifier error
1 MHz, typical voltage noise level is 7 NVQQ screenshot 20141204111321.jpg. In addition to suppressing radio frequency interference, the filter has a input overload protection function. Because resistors R1a and R1B help isolate the instrument amplifier input circuit and external signal source.
Fig. 2 is a simplified diagram of the Anti-RF interference circuit. As can be seen from the figure, the filter forms a bridging circuit that outputs an input pin that is connected to the instrument amplifier. In view of this connection method, any mismatch between the two time constants between C1A / R1A and C1B / R1B can cause bridge failure, thereby reducing high frequency common mode suppression performance. Therefore, resistors R1a and R1B and capacitance C1a and C1b should always be equal.
Figure 2 The capacitor C2 constitutes the bypass of C1A / C1B, and can effectively reduce the exchange common mode suppression error caused by the mismatch of components.
As shown, C2 is connected to the output of the bridge, so that C2 actually has a parallel relationship with the series combination composed of C1a and C1b. After this connection, C2 can effectively reduce any communication common mode suppression error due to mismatch. For example, if the C2 is 10 times larger than C1, this connection method will reduce the common mode suppression error caused by the mismatch of the C1a / C1b to be reduced to one-third of the original. It should be noted that the filter does not affect the common mode suppression of DC.
RF interference suppression circuit for AD620 series instrument amplifier
Figure 3 is a circuit for universal instrument amplifiers, such as the AD620 series, higher than the AD8221 series (12 NVHz), low bandwidth. Accordingly, such instrument amplifiers use the same input resistance, but the value of capacitor C2 increases by about 5 times, reaching 0.047 f to provide sufficient radio frequency attenuation. When using the value shown in the figure, the circuit is about 400 Hz; by falling R1 and R2 to 2.2 k, the bandwidth can be increased to 760 Hz. It should be noted that the increase bandwidth is the cost of paying The circuit drive of the previous circuit drive required for the instrument amplifier is low, so the input overload protection performance is lowered to some extent.
Figure 3 Radio frequency interference suppression circuit for the AD620 series meter amplifier
RF interference suppression circuit for micro-power meter amplifier
Some meter amplifiers are more prone to radio streams than other amplifiers, thus need to adopt stronger filters. The input-stage working current is a low power meter amplifier (such as AD627) is a good example. Increasing the value of the two resistor R1A / R1B and / or the value of the value of the capacitor C2 can increase the radio frequency attenuation, but the cost is a decrease in signal bandwidth. Since the AD627 meter amplifier has higher noise (38nv Hz) compared to the general-purpose integrated circuit (such as the AD620 series device), an input resistor having a higher resistance value can be used without the noise performance of the circuit. Figure 4 modifications to the basic RC Anti-RF interference circuit shown in FIG. 1, using a higher input resistance with a higher resistance value.
Figure 4 RF interference suppression circuit for AD627
The filter band is about 200 Hz. When the gain is 100, when the input is 1V P-P, the maximum straight outlet voltage is about 400 VRTI within 1 Hz to 20 MHz frequency range. When the gain does not change, the radio frequency signal suppression (output terminal radio frequency level / input radio frequency level) of the circuit will exceed 61 dB.
RF interference filter for AD623 meter amplifier
Figure 5 shows the Anti-RF interference circuit used to be used with the AD623 instrument amplifier. Since such a device is compared with the AD627, it is more difficult to be affected by RF interference, so the value of the input resistance can be reduced from 20 k to 10 k, and the result will increase the signal bandwidth of the circuit and reduce the noise contribution of the resistance. In addition, 10K resistors provide extremely effective input protection. When using the value shown in the figure, the filter has a bandwidth of about 400 Hz. When the gain is 100, when the input is 1 Vp-P, the maximum straight-out loss voltage is less than 1 V RTI. When the gain does not change, the radio frequency signal of the circuit is better than 74. DB.
Figure 5 AD623 RF interference suppression circuit
AD8225 RF interference filter circuit
Figure 6 shows a recommended RF interference filter for such an instrumental amplifier. The AD8225 meter amplifier is fixed to 5, and the AD8221 is more vulnerable to RF interference. If a radio frequency interference filter is not used, the straight outlet power measured by this instrument amplifier is about 16 mV RTI when input 2 VP-P, 10 Hz to 19 MHz sine waves. By using a resistance of the resistance value, the filter can obtain a higher radio frequency attenuation than the AD8221 circuit: replace 4 k with 10 k. Since the AD8225 has a high noise level, this is acceptable. If a filter is used, the straight-conversion voltage error is negligible.
Figure 6 AD8225 RF Interference Filter Circuit
Make an instrument amplifier RF interference filter using a common-mode frequency choke circle
As an alternative to the RC input filter, a commercial common-mode frequency choke can be connected in front of the meter amplifier, as shown in Figure 7. The common mode choke is a dual winding radio frequency choke with a shared iron core. Any common mode input radio frequency signal of the two inputs will be attenuated by the choke. The common-mode choke provides a small amount of radio frequency interference suppression in a small amount of component, and it has obtained a wider signal passband, but the effectiveness of this method depends on the quality of the common mode choke used, it is best to use Internal matches a good choke. Another potential problem using the choke is unable to increase the input protection function as the RC radio frequency interference filter. When the radio frequency choke is used, the rated gain is 1000 AD620 meter amplifier, and when inputs 1 VP-P common mode sine wave, the circuit shown in Fig. 7 can reduce the straight outlet power to less than 4.5 V RTI level. The high frequency common mode inhibition ratio is also reduced, as shown in Table I.
Figure 6 AD8225 RF Interference Filter Circuit
Make an instrument amplifier RF interference filter using a common-mode frequency choke circle
As an alternative to the RC input filter, a commercial common-mode frequency choke can be connected in front of the meter amplifier, as shown in Figure 7. The common mode choke is a dual winding radio frequency choke with a shared iron core. Any common mode input radio frequency signal of the two inputs will be attenuated by the choke. The common-mode choke provides a small amount of radio frequency interference suppression in a small amount of component, and it has obtained a wider signal passband, but the effectiveness of this method depends on the quality of the common mode choke used, it is best to use Internal matches a good choke. Another potential problem using the choke is unable to increase the input protection function as the RC radio frequency interference filter. When the radio frequency choke is used, the rated gain is 1000 AD620 meter amplifier, and when inputs 1 VP-P common mode sine wave, the circuit shown in Fig. 7 can reduce the straight outlet power to less than 4.5 V RTI level. The high frequency common mode inhibition ratio is also reduced, as shown in Table I.
Figure 8 Typical settings for measuring measuring measurements for instrumental amplifier radio frequency interference
Take an oscilloscope to adjust the sine wave generator to make the output of the generator to 1 V P-P. Set the meter amplifier to high gain (eg 100). Direct loss voltages can be read directly on the output of the meter amplifier. To measure the high-frequency common mode suppression, the oscilloscope is connected to the meter output terminal with the meter amplifier output, and measure the peak output voltage of the input frequency (in place). When calculating the common mode suppression ratio of the frequency, it is important to consider the input end (VIN / 2) and the gain of the meter amplifier.
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