Summary of Simple circuit provides precision ADC interface
This article describes the AD628 precision-gain-block IC, configured as a differential-input amplifier and two-pole lowpass filter. It extracts weak signals riding on high common-mode voltages up to ±120V, overcoming limitations of traditional discrete op-amp circuits like poor common-mode rejection and board space consumption. The circuit operates on dual supplies from ±2.25V to ±18V or single supply with VREF biasing, directly driving an ADC while providing offset capability.
Parts used in the AD628 Precision-Gain-Block Circuit:
- AD628 precision-gain-block IC
- External capacitor C1
- Internal 10-kΩ resistor at output of A1
- External resistor RF
- External resistor RG
- External capacitor C2
- Analog-to-Digital Converter (ADC)
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.
AnAD628precision-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 theAD628output 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.
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- What voltage range can this circuit handle for common-mode signals?
The circuit can extract weak signals riding on common-mode voltages as high as plus-minus 120 volts. - How does the AD628 improve upon traditional instrumentation amplifiers?
It overcomes poor common-mode rejection, input voltage range limits, gain drift, and saves board space by integrating filtering on one chip. - Can the circuit operate with a single power supply?
Yes, you can bias the VREF pin at VS divided by 2 to allow single-supply operation. - What determines the first pole of the lowpass filter?
The first pole is a function of the internal 10-kΩ resistor at the output of amplifier A1 and the external capacitor C1. - How is the second pole created in this design?
The second pole is created by an external RC time constant comprising capacitor C2 across resistor RF in the feedback of amplifier A2. - Why use a two-pole filter instead of a single-pole filter?
The second pole provides a more rapid roll-off of frequencies above its corner frequency compared to a single-pole lowpass filter. - What happens to the gain of amplifier A2 at high frequencies?
As input frequency increases, the gain of amplifier A2 eventually drops to unity and does not decrease further. - How is the voltage gain set for frequencies below the corner frequency?
The ratio of RF to RG sets the voltage gain of amplifier A2 at frequencies below its minus 3-dB corner.
