High Performance Differential Amplifier with Precision Supply Reference Level Shifting

High-performance ADCs designed with small-size processes are usually powered by a single 1.8V to 5V power supply. To process signals of ±10 V or greater, an amplifier circuit is usually placed in front of the ADC to attenuate the signal and prevent input saturation. A differential amplifier (diff amp) is commonly used when the signal contains a large common-mode voltage.

The ability of a differential amplifier to reject common-mode voltages is determined by the ratio matching of the gain-setting resistors; the better the match, the higher the common-mode rejection ratio (CMR). For discrete amplifiers using 0.1% external resistors, the CMR is limited to 54 dB. ICs that integrate tightly laser-trimmed resistors and op amps can achieve CMRs greater than 80 dB.

Like many other analog ICs, early differential amplifiers were typically powered by dual supplies of ±5V to ±15V. As ADCs and other components moved toward lower supply voltages, for a while the differential amplifier was the only circuit in the front end that required dual supplies. But adding a negative supply to this one circuit is rather inconvenient.

New differential amplifiers can operate from a single 2.7V to 15V supply, but under certain operating conditions, the op amp inputs and outputs are all tied to the negative rail (ground). To measure signals containing negative common-mode voltages, the common-mode inputs must be raised off the negative rail. To measure negative signals, the amplifier outputs must be raised off the negative rail. Both of these level shifts can be accomplished by applying a negative voltage to the reference pin. For example, with a single 5V supply, a 2.5V source at the reference pin sets the output to midsupply and will raise the common-mode voltage seen at the op amp inputs. The supply must be low impedance to avoid degrading CMR and low drift to maintain accuracy over temperature. Figure 1 shows a typical solution using two external precision resistors and a low-drift precision op amp.

Figure 2 shows an alternative solution that uses the AD8271 differential amplifier with multiple precision-trimmed resistors integrated on the amplifier to achieve lower cost and higher performance. The on-chip resistors set the device output to midsupply. These resistors are all made of the same low-drift thin-film material, so their ratios are very well matched over temperature; they are trimmed to match the other resistors in the circuit, which does not degrade the excellent CMR performance.

Precision Programmable Gain Differential Amplifier

The AD8271 low-distortion, programmable-gain differential amplifier contains a precision op amp and seven laser-trimmed gain-setting resistors to achieve user-selectable differential gains of 0.5, 1, or 2. It can also be configured in more than 40 single-ended configurations with a gain range of -2 to +3. The device is available in two grades: the B-grade features are specified as a maximum gain error of 0.02%, a maximum gain drift of 2ppm/°C, a maximum offset voltage of 600μV, and a minimum common-mode rejection ratio of 80dB; the A-grade features are specified as a maximum gain error of 0.05%, a maximum gain drift of 10ppm/°C, a maximum offset voltage of 1000μV, and a minimum common-mode rejection ratio of 74dB. Both grades feature –110dB of harmonic distortion, 15MHz bandwidth, and a slew rate of 30V/us. This combination of speed and precision makes the device ideal for instrumentation amplifiers, driving ADCs, level shifting, and automatic test equipment. The AD8271 operates from a 5-V to 36-V single supply or a ±2.5-V to ±18-V dual supply and consumes 2.3 mA. It is available in a 10-lead MSOP package and is specified over the –40°C to +85°C temperature range. It is priced at $1.25 per unit in 1,000-unit quantities.

Figure 1: The AD8271 converts the output to an intermediate supply without external components.