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Intro

The schematics linked below describe a differential input, single ended output buffer in the configuration known as an instrumentation amp. This design utilizes a device from Texas Instruments (former Burr-Brown division) which uses an op-amp (their OPA-134 design) in a difference amplifier configuration with included on-chip resistors laser trimmed to precisely match the resistance ratio. Implementing the equivalent circuit used to be difficult for the hobbyist because of the close resistor matching required. Digikey now sells 0.1% resistors, so the task is easier, but the TI devices are still more economical in small quantities, and of course the construction labor is reduced.

Circuit description

The basic instrumentation amp is a differential amplifier configuration which is fed by a pair of buffers to present a high common mode impedance to the source and a low impedance drive to the differential amplifier.

Some audio equipment uses a differential amplifier connected directly to the input connector, without the buffers which are used in the instrumentation amplifier configuration. The disadvantage of using a differential amp with no buffers is that the common mode rejection ratio is dependent on the precise matching of source impedance of each leg of the component driving the input. If you look at the circuit configuration, you can see why this is the case: CMRR is dependent on the ratio match of the resistors around the op-amp, and the source impedance of the driving component becomes part of the resistance connected to the input terminals of the op-amp.

The bias resistors that connect to the inputs of the buffers should be matched as well, but since the impedance is so high, matching is much less critical. The output impedance of the source forms a voltage divider with the input impedance of the buffer, with the common mode voltage (ground potential difference between the source and destination device) forming the source for both paths. Any differences in voltage at the two inputs (hot and cold of the balanced input) due to impedance imbalance is differential to the input at that point, and cannot be cancelled by the second stage difference amp. Using 1M Ohm input bias resistors, if the two inputs are each off by 1% (i.e. one leg is 1% high and one leg is 1% low), and the output device is driven by 100 Ohm output impedance, also mismatched by 1% on each leg, the resulting mismatch will be over 100dB below the level of the common mode signal. Real world circuits are not likely to achieve this level of common mode rejection due to imbalances elsewhere, but that just gives an idea of how little CMRR degrades with even 1% resistors when using the instrumentation amplifier configuration.

Schematics

input buffer on crossover Here are two schematics of the input buffer circuit, one a single channel version which I built for the input of my Phoenix crossovers, and one a two channel version which I built for the inputs to a WM-1 circuit board. Only the bypass capacitors local to each chip are shown, additional larger bypass capacitors may be needed where the power connection enters the circuit, depending on the physical placement of this circuit. No DC blocking capacitors are shown. In my system they are not needed, you will have to decide whether or not to include them in a different implementation. Size the blocking capacitors larger than you need in order to keep the effect of the input capacitors on input impedance match as small as possible.

Balanced to SE buffer

A two channel version

The input op-amps are Texas Instruments (formerly Burr-Brown) OPA-2134. You can buy them from Digikey, part number OPA134PA-ND for the DIP version, OPA134UA-ND for the SO-8 package. The difference amplifier for the single channel circuit is INA-137, Digikey part INA137PA-ND for the DIP version. The two channel circuit uses the INA-2137, Digikey part INA2137PA-ND.

Note that the INA-137 and INA-2137 parts have a gain of 0.5, i.e. a 6dB drop relative to the input. This works out well as an input buffer to the Phoenix crossover since that circuit assumes a 6dB drop in the input due to the voltage divider formed by the input resistors. Using the '137 based circuit, you can take the 6dB loss in the input buffer and replace the 5k Ohm resistor to ground (R4) in the Phoenix circuit with a 100k Ohm to ground. The output of the buffer can be connected to the pads for C3.

top side of RF filter bottom side of RF filter The RFI filter in the Phoenix crossover design should be built as two indentical RC networks on each input leg of the buffer amp. The RFI filter in a balanced configuration should have a low inductance connection between the reference node of the filter and the shield ground. My implementation uses surface mount capacitors for best high frequency performance, and the ground plane of the circuit is soldered to a wide sheet of tin which is clamped to the chassis by the input connectors.

If you do not want the 6dB drop handled in the buffer stage for some reason, substitute the INA-134 (INA-2134) in place of the INA-137 (INA-2137).

Connections (balanced and unbalanced)

Balanced input connections should fairly obvious. Current international agreement is that pin 2 of an XLR connector is considered the positive signal polarity, although you can of course feel free to use the "pin 3 hot" signal convention if you need to interface with older equipment for some reason.

Unbalanced outputs should be connected with shielded twisted pair cable, with the center connector of the source connected to the positive polarity input on the balanced input end, the ground of the source connected to the negative polarity input on the balanced end, and the shield connected to the shield/shell on both ends. This connection is equivalent to a balanced connection with a source impedance imbalance equal to the output impedance of the unbalanced source, so the lower the source impedance, the better the noise rejection will be.

A better solution is to modify the output of the unbalanced source, by first lowering the output impedance if it is above 100 Ohms, and then creating a second output connection by connecting an identical impedance to the circuit ground reference, ideally as close to the ground reference of the output buffer as possible.

The 100K bridging resistor is not really needed, and can be left out if desired. If you use the RC lowpass RFI filter from the Phoenix crossover schematic, you will have an extra 0.82 dB of attenuation due to the resistor divider action. If you are using this buffer in an application which needs exactly unity gain or exactly -6dB gain for some reason, either leave out the 100K bridging resistor, or place the resistor before the RF filter.

If a balanced output from the buffer is desired, use two difference amplifier stages with a single pair of buffers. Connect the two difference amplifiers with opposite polarity so that the outputs are anti-phase. In that configuration, using an INA2137 to provide the two difference stages will give unity gain, and an INA2134 will give you 6dB of gain.

References


Page last updated: 16 July 2002


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