Instrumentation amplifier circuitry which has incorporated low-cost general-purpose op amps provides the designer with economical, quality performance options. Improved instrumentation amplifier circuits are possible because of the development of op amps using junction FETs. These op amps have improved input impedance characteristics and ac performance compared with general purpose bipolar devices. Metal gate CMOS op amps have reduced the power required and will operate at voltages as low as 2V.
Because the input offset voltage of a metal gate CMOS circuit changes with varying differential input voltage levels, there are severe drawbacks in using this technology for op amps. LinCMOS technology overcomes these disadvantages. LinCMOS devices do not have an input offset shift with differential input voltage and can operate satisfactorily down to 1.0 V supply. TLC271 LinCMOS op amps are used in the instrumentation amplifier illustrated in Figure 3-40 because of their unique features. Some of these features are:
- Operate at low voltages
- Input signal operation close to -VCC rail
- Reasonable ac performance at low power
- Provide the high input impedance characteristic of FET input devices
- Offset stability
- High CMRR
- Power/performance adjustment for desired performance levels while maintaining the lowest possible power requirement.
The TLC271 operational amplifiers (the first monolithic devices to combine these characteristics) allow the construction of a ± 5-V instrumentation amplifier with reasonable ac performance. Some of the important features of Figure 3-40 which should be pointed out are:
- Three op amps U1, U2, and U3 are connected in the basic instrumentation amplifier configuration.
- Operating from ± 5 V. pin 8 of each op amp is connected directly to ground and provides the ac performance desired in this application (high bias mode).
- Two adjustment pots are used. P1 is for offset error correction and P2 allows adjustment of the input common mode rejection ratio.
- The high input impedance allows metering of signals from sources of several megohms without loading. The resulting circuit frequency response is 200 kHz at – 3 dB and has a slew rate of 4.5 V/µs. This is a significant improvement over general bipolar performance. The signal response and speed characteristics are particularly significant in light of the low supply voltage and supply currents. Total supply current is 670 µA per supply.
- Output error voltages of less than 1 % are experienced over the 0oC to 70°C operating range.