High Speed Instrumentation Amplifier Using the AD8271 Difference Amplifier and the ADA4627-1 JFET Input Op Amp
This circuit uses a monolithic difference amplifier with laser trimmed thin film resistors for the output amplifier, thereby providing good dc and ac accuracy with fewer components than the traditional approach.
Figure 1. In Amp with Gain = 201 (Simplified Schematic: Decoupling and All Connections Not Shown)
The op amps selected for the input stage must also have low offset voltage and low offset voltage drift with temperature. They also need to have good drive characteristics. This allows the use of low value resistors to minimize resistor thermal noise.
Headroom issues relating to the op amp must be considered in this circuit for proper operation.
When working with any op amp having a gain-bandwidth product greater than a few MHz, careful layout and bypassing are essential. A typical decoupling network consists of a 1 μF to 10 μF electrolytic capacitor in parallel with a 0.01 μF to 0.1 μF low inductance ceramic MLCC type.
For the lowest noise with low impedance sources only, low voltage noise is important. The AD8599 has lower noise, lower offset voltage drift, and lower supply current; but the input bias currents are much higher, and the bandwidth will be lower than that obtained with the ADA4627-1. The measured −3 dB points are 56.6 kHz and 87.6 kHz for the AD8599 and ADA4627-1, respectively. (See Figure 2).
Figure 2. Bandwidth of Circuit Shown in Figure 1 Comparing the ADA4627-1 to the AD8599 as the Input Stage.
With high impedance sources, the input bias current and the input noise current of a bipolar op amp can result in errors. The bias current creates an I × R drop, which will be multiplied by the overall circuit gain. This can result in several volts of offset at the output. The input noise current is also multiplied by the source impedances, creating an additional noise voltage. To avoid this, a JFET input op amp, such as the ADA4627-1, should be used. Even though the voltage noise is slightly higher than the AD8599, the current noise is significantly lower, resulting in lower overall noise when used with high impedance sources.
As Figure 3 and Figure 4 show, the AD8599 is the proper choice with low source impedances, and the ADA4627-1 is better with higher source impedances. There is a trade-off: the input capacitance of JFET op amps is higher than bipolar op amps, so the RC time constant must be considered.
Figure 3. Noise Spectral Density (RTO) of Circuit Shown in Figure 1 Comparing the ADA4627-1 to the AD8599 as the Input Stage: Low Impedance Source (0 Ω)
Figure 4. Noise Spectral Density (RTO) of Circuit Shown in Figure 1 Comparing the ADA4627-1 to the AD8599 as the Input Stage: High Impedance Source (66 kΩ)
For the lowest possible drift over temperature, one of the auto-zero amplifiers, such as the AD8539, can be used, but the bandwidth will be reduced and wideband noise increased. This would be an excellent choice for bandwidths less than 10 Hz, however.
When selecting op amp and difference amplifier combinations for this circuit, always ensure that the input common-mode voltage range of each amplifier is not violated. This is commonly overlooked but is the subject of a fair number of application questions.
If the first stage gain is greater than about five, consider using a decompensated op amp, such as the OP37, to get a higher slew rate and signal bandwidth with less supply current. To avoid common- mode oscillation, the circuit must be modified slightly as described in "Phase Compensation of the Three Op Amp Instrumentation Amplifier." White, D. Rod. IEEE Transactions on Instrumentation and Measurement. Vol. IM-36, No. 3, Sept. 1987.
With microvolt-level input signals and a gain of 1000, the first stage can be operated on ±2.5 V, saving power and giving more choices of op amps, such as the AD8539 auto-zero amplifier. However, if the input common-mode voltage range is high, an op amp with a higher supply voltage must be chosen for the first stage.
|AD8274||Very Low Distortion, Precision Difference Amplifier||
|AD8271||Programmable Gain Precision Difference Amplifier||
|ADA4627-1||36 V, 19 MHz, Low Noise, Low Bias Current, JFET Op Amp||
|AD8599||Ultralow Distortion, Ultralow Noise Op Amp (Dual)||