High Speed FET Input Instrumentation Amplifier with Low Input Bias Current and High AC Common-Mode Rejection (CN0273)
The circuit shown in Figure 1 is a high speed FET input, gain of- 5 instrumentation amplifier (in-amp) with a wide bandwidth (35 MHz) and excellent ac common-mode rejection, CMR, (55 dB at 10 MHz). The circuit is ideal for applications where a high input impedance, fast in-amp is required, including RF, video, optical signal sensing, and high speed instrumentation. The high CMR and bandwidth also makes it ideal as a wideband differential line receiver.
Most discrete in-amps require expensive matched resistor networks to achieve high CMR; however, this circuit uses an integrated difference amplifier with on-chip matched resistors to improve performance, reduce cost, and minimize printed circuit board (PCB) layout area.
The composite in-amp circuit shown in Figure 1 has the following performance:
Figure 1. High Speed FET Input Instrumentation Amplifier (Note: Power Supply Decoupling Not Shown)
Most fully integrated in-amps are fabricated on bipolar or complementary bipolar processes and are optimized for low frequency applications with high CMR at 50 Hz or 60 Hz. However, there is a growing need for wide bandwidth in-amps for video and RF systems to amplify high speed signals and provide common-mode rejection of unwanted high frequency signals.
When a very high speed, wide bandwidth in-amp is needed, one common approach is to use two discrete op amps with high input impedance to buffer and amplify the differential input signal in the first stage, and then configure a single amplifier as a difference amplifier in the second stage to provide a differential-to-single-ended conversion. This configuration is known generally as a 3-op-amp in-amp. This approach requires four relatively expensive precision-matched resistors for good CMR. Errors in matching produce errors at the final output.
The circuit shown in Figure 1 solves this problem by using the ADA4830-1 integrated high speed difference amplifier. The laser-trimmed thin film resistors are matched to very high precision, thereby eliminating the need for four relatively expensive precision-matched external resistors.
In addition, the use of the high speed, dual ADA4817-2 as the input stage amplifier allows the composite in-amp to provide a bandwidth as high as 80 MHz when the overall gain of the circuit is 2.5.
The use of the dual ADA4817-2 amplifiers in a single 4 mm × 4 mm LFCSP package and the integrated ADA4830-1 difference amplifier significantly reduces board space, thereby reducing design costs for large systems.
The circuit can be used in noisy environments because both the ADA4817-2 and ADA4830-1 offer low noise and excellent CMR performance at high frequencies.
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