Low power difference amps claim small size and supreme speed

Used to reject external interferences in low-power systems, single-chip difference amplifiers have for some time provided smaller, often-simpler alternatives to their discrete counterparts. Now, Analog Devices' (ADI) claims size and speed advantages for its latest AD827x family of difference amplifiers over discrete and non-discrete amplifier technologies alike.

The 4.9 x 3mm AD827x family, which are half the size of competing devices according to ADI, boast slew rates up to 10 times faster than traditional difference amplifiers. With a bandwidth of 550kHz and a slew rate of 1.1V/µs (volts per microsecond), plus an input range of two times the power supply (which has a range of 2.5 to 36V), the AD827x family particularly suits voltage and current sensing in process-control, motor-control, and power-management applications.

Asked how the slew rates advertised have been achieved, Moshe Gerstenhaber, Design Engineering Manager of ADI's Industrial and Instrumentation group and an ADI Fellow responded: “To achieve high slew rate one can degenerate the input stage. This results in the desired slew rate, but the noise goes up as well as the drift and the gain of the amplifier is reduced. We developed a non degenerated input stage with a linear Iout/Vin relationship to achieve about a 10x improvement to the slew rate/bandwidth.”

These amplifiers have a 220µA maximum quiescent current and 86dB CMRR (common-mode rejection ratio), cited as 10dB more than competing devices. Another feature is the 15¼S settling time to 0.01 percent – an important metric, as faster settling times ensure shorter propagation delay in control loops. Additionally, on-chip resistors are laser-trimmed for accurate gain, low gain drift, and high common-mode rejection.

The AD8276 difference amplifier can be used to create a current source, in combination with ADI's AD8603 op amp. Further details on how to achieve this and other circuits designed by ADI's application engineers can be found in ADI's Circuits from the Lab collection.

An additional note about calculating slew rates from Moshe:

“Slew rate of a signal is a function of the amplitude one tries to produce, and the frequency that one tries to reproduce. Specifically, a 10V amplitude at 1MHz requires about 70V/µsec slew rate. One can easily calculate the required rate of change as a function of amplitude and frequency, i.e. 500KHz and 10V about 35V/µsec, 1V at 500KHz 3.5V/µsec, and 0.1V at 500KHz will work well at 1.1V/µsec. So a 0.1V peak signal (0.2V peak to peak) will be amplified at 550KHz with little problem.”

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