Difference amps keep up with high-potential current affairs
-- EDN, January 20, 2000
When you need to sense a small voltage across a current-sense resistor that has both of its terminals at a high voltage with respect to ground or extract a small difference voltage despite a large common-mode voltage, you have a simple-in-principle, difficult-in-practice design challenge. Three devices from three vendors ease this problem. Linear Technology's LT1787 current-sense amplifier works with supplies as high as 60V for sensing bidirectional currents from microamps through kiloamps, depending on resistor value. Its 40-µV input offset voltage means minimal measurement-accuracy compromise. With 12-bit dynamic range, the device provides minimum resolvable current that is 1/10,000 the full-scale current value. It has a fixed gain of eight, which the vendor sets by onboard precision resistors, and you can implement lowpass input-noise filtering by adding an external capacitor. Input-voltage range for this $2.05 (1000) SO-8 amp is 2.5 to 60V, and operating current is 60 µA.
Burr-Brown's INA145 and INA146 difference amplifiers support programmable gain values of 0.1 to 100V/V, which you set via external resistors. The INA146 is for ±100V differential and common-mode inputs, with 86-dB common-mode-rejection ratio (CM-RR), and the INA145 is for ±28V inputs and has 80-dB CMRR. Both SO-8 devices use single or dual supplies of 4.5 to 36V and feature gain error of 0.025%. The INA45 costs $1.40 (1000), and the INA146 costs $1.60.
For higher common-mode voltages, the AD629 difference amplifier from Analog Devices may be a good choice. This amplifier works in situations with common-mode voltages as high as ±250V and has an operating range that you can use with transients as high as ±500V. Minimum CMRR is 77 to 86 dB, depending on grade. The AD629 is available in eight-pin DIP or SO packages, and prices begin at $4 (1000).
Analog Devices Inc, www.analog.com/inamps.
Burr-Brown Corp, www.burr-brown.com.
Linear Technology Corp, www.linear-tech.com.
-by Bill Schweber
