Isolation Amplifier - Amplifier Selection Guidelines

Amplifier Selection Guidelines

Instrumentation amplifiers can be classified into four broad categories, organized from least to most costly:

• Single-ended. An unbalanced input, non-isolated. Suitable for measurements where common mode voltages are zero, or extremely small. Very inexpensive.

• Differential. A balanced input, non-isolated. Suitable for measurements where the sum of common mode and normal mode voltages remains within the measurement range of the amplifier.

• Single-ended, floating common. An isolated and quasi-balanced input (the floating common is typically connected to the (-) input of a differential amplifier). Suitable for off-ground measurements up to the breakdown voltage of the isolation barrier, and exhibits very good common mode rejection (100 db typical).

• Differential, floating common. An, isolated and balanced input. Suitable for off-ground measurements to the breakdown voltage of the isolation barrier, and exhibits superb common mode rejection (>120 db).

For most industrial applications that require isolation, the single-ended floating design provides the best price/performance.

There are also two broad classifications of isolation amplifiers that should be considered in tandem with the application:

• Amplifiers providing input-to-output isolation without channel-to-channel isolation. This is a less expensive form of isolation that offers only one isolation barrier for a multi-channel instrument. Although the commons of each channel are isolated from power ground by the input-to-output isolation barrier, they are not isolated from each other. Therefore a common mode voltage on one will attempt to float all the others, sometimes with disastrous results. This form of isolation is suitable only when it is certain that that there is only one common mode voltage that is equally applied to all channels.

• Amplifiers providing both input-to-output and channel-to-channel isolation. This is the purest form of isolation, and the option that should be considered for nearly all applications. Multi-channel instruments that employ it are immune to inconsistent common mode voltages on any combination of channels within the limits of the amplifiers.