Perform audio line receiver impedance balancing
Keywords:single op-amp differential amplifier? audio balanced line receiver? common-mode rejection?
Audio line receiver ICs, with laser trimmed on-chip resistors (such as the unity-gain THAT1240/INA134 or "-6dB" attenuating THAT1246/INA137) have excellent common-mode rejection, typically 90 dB. Real world audio interconnections are not always balanced and are frequently a combination of unbalanced single-ended, balanced, floating and ground-referred sources.
The simple differential amplifier topology of figure 1, whether made from op amps using external precision resistors or readily-available line receiver ICs, has two subtle limitations that prevent it from being truly universal.
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Figure 1: The "unity gain" audio line receiver with internal laser-trimmed resistors is a simple differential amplifier. |
Simple diff amps have unpredictable input impedance and gain
The first limitation is the unpredictable input "port" impedances to ground at each input. The impedance at the inverting (-) input is voltage-dependent on the non-inverting (+) input. Depending on the type of connections to the inputs, (e.g., unbalanced single-ended, balanced ground-referred, or fully-floating) the apparent impedance at the inverting input can vary over a wide range. An extreme example is when a grounded centre-tap transformer drives both inputs with equal but opposite polarities.
In a unity-gain line receiver (where R1=R2=R3=R4) the signal current in the inverting input is three times the non-inverting input current [1,2,3]. Fortunately, the impedance for common-mode signals (both inputs driven equally) are identical at each input port or the circuit in figure 1 would perform poorly when rejecting hum and interference in the real world.
A second limitation is unequal gain when fed by single-ended sources that have one input connected and the other left floating or "open port." Typical open-port examples are a threeconductor 1/4" (TRS) plug with the "ring" terminal unconnected or an XLR to RCA phono plug adapter cable with either pin 2 or 3 open. In figures 2a and 2b, a unity-gain line receiver is shown.
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Figure 2: Gain variations occur using the circuit of figure 1 when one input is driven and the opposing input is "open port." Figures 2a and 2b show unity-gain line receivers; 2c and 2d are -6dB examples. |
Driving the inverting input while leaving the non-inverting input open provides the expected gain of 0 dB. However, reversing the input connections (feeding the non-inverting input with the inverting input open) provides 6dB attenuation because there is no ground connection for the inverting feedback network to provide 6 dB of gain to negate identical attenuation at the non-inverting input.
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