Real-world measurement requires the extraction of weak signals from noisy sources. High common-mode voltages are often present even in differential measurements. The usual approach to this problem is to use an op amp or an instrumentation amplifier and then perform some type of lowpass-filtering to reduce the background noise level. The problems with this traditional approach are that a discrete op-amp circuit has poor common-mode rejection, and its input voltage range is always lower than the power-supply voltage. When you use a differential signal source with an instrumentation-amplifier circuit, using a monolithic IC can greatly improve common-mode rejection. However, a standard instrumentation amplifier cannot handle sources greater than the power-supply voltage or signals riding on high common-mode voltages. Instrumentation amplifiers using a single external gain resistor also suffer from gain drift. In addition, lowpass filtering requires the use of a separate op amp along with several external components. This approach uses up valuable board space. The circuit of Figure 1 overcomes all of these performance limitations on one µSOIC.
An AD628 precision-gain-block IC is configured as a differential-input amplifier and a two-pole lowpass filter. This circuit can extract weak signals riding on common-mode voltages as high as ±120V. The precision-gain block directly drives an ADC. A separate VREF pin is available for offsetting the AD628 output signal so that it is centered in the middle of the ADC's input range. Although Figure 1 indicates ±15V, the circuit can operate with ±2.25 to ±18V dual supplies. The VREF pin can also allow single-supply operation; for this purpose, you simply bias VREF at VS/2. The gain block has two internal amplifiers: A1 and A2. Pin 3 connects to ground, thus operating amplifier A1 at a gain of 0.1. The output of A1 directly drives the positive input of amplifier A2.
The first pole of the lowpass filter is a function of the internal 10-kΩ resistor at the output of A1, and an external capacitor, C1. The gain of A2 is a function of external resistors RF and RG. An external RC time constant in the feedback of A2 creates the second pole. This time constant comprises capacitor C2 across resistor RF. Note that this second pole provides a more rapid roll-off of frequencies above its RC “corner” frequency (1/(2πRC)) than does a single-pole lowpass filter. However, as the input frequency increases, the gain of amplifier A2 eventually drops to unity and does not decrease. So, the ratio of RF/RG sets the voltage gain of amplifier A2 at frequencies below its –3-dB corner and unity gain at higher frequencies.
Figure 2 is a graph of the filter's output versus frequency using components to provide a 200-Hz, –3-dB corner frequency. Note the sharp roll-off between the corner frequency and approximately 10 times the corner frequency. Above this point, the second pole starts to become less effective, and the rate of attenuation is close to that of a single-pole response. Table 1 and Table 2 provide typical component values for various –3-dB corner frequencies and two full-scale input ranges. The values are rounded off to match standard resistor and capacitor values.
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