Additional stages can be added, but may introduce errors
The classic three op-amp instrumentation amplifier (not shown) eliminates the problem of signal and source-dependent port impedance but has other audio performance issues in addition to increased component count. "Two op-amp" approaches to impedance balance inputs can introduce signal path asymmetry. Figure 3 provides one example .
Figure 3: Example of an impedance-balanced line receiver made with an NE5532 op-amp. For -6dB attenuation R1=R2=R3=R4. C3, typically 10-33 pF, maintains stability. C1 and C2 prevent severe transient overshoot. Nine external passive components are required. With 1% resistors, the common-mode rejection ratio is typically 40 to 50 dB. A useful feature of this circuit is that the noise contribution of A2 appears in common mode at the balanced output.
The lower end of R4, normally grounded in a simple differential input, is now driven by an inverted copy of the output to lower, or "anti-bootstrap" the apparent impedance of the non-inverting input to make its port impedance to ground equal to that of the inverting input. With balanced sources, input impedance balance in Figure 3 results from A1's inputs being held in null at virtual ground by stage A2. Due to the subtractive process, the overall gain is -6dB.
Asymmetry exists in Figure 3 due to unavoidable delay in subtractive stage A2 which reduces common-mode rejection at high frequencies. Another significant issue stemming from A2's delay is that large transient overshoots (easily more than 50%) occur with fast-rise-time signals because subtractive feedback from A2 arrives too late . To prevent overshoot, stage A1 must be made significantly slower than A2 requiring three external capacitors.
A greater disadvantage of this and other approaches is that they require off-chip components whose tolerances degrade common mode rejection. Commonly available line receiver ICs, with their highly desirable matched on-chip resistors, do not allow direct access to the op-amp inputs. Capacitors cannot be connected to the internal op-amp inputs to control transient response due to pin-out restrictions.
"Double-balanced" cross-coupled inputs are both impedance-balanced and symmetrical
Figure 4 shows an improved two op-amp circuit, using a "double-balanced," fully symmetrical, cross-coupled topology that provides both impedance and level balanced inputs along with high common mode rejection [5,6,7]. In this example, a dual THAT1286 (or INA2137) -6dB line receiver is chosen to accept the +27 dBu maximum inputs required to provide "headroom" in professional audio applications in which ±15V supplies are used.
Figure 4: The double-balanced cross-coupled input equalizes port impedances to ground within a fraction of one percent and reduces variations in gain to <<0.5dB when one input is "open port." Laser-trimmed resistors and symmetry provide common-mode rejection ratios that are typically 90 dB using no external components. When the balanced output is used, the finite HF common-mode rejection limitations of op amps A1 and A2 attempt to cancel.
By cross-coupling opposing inputs, the unequal input impedance to ground for each input polarity appear in parallel to an opposing amplifier. The impedance differences equalize, forcing the impedance to be the same for each input.
One curious aspect of this circuit is that for single-ended outputs, one output is unused. At first glance it might appear that this stage has no function, but closer inspection reveals that the output modifies input impedance by driving current through the internal feedback network. The cross-coupled inverting input of one stage lowers impedance at the opposite stage's non-inverting input using anti-bootstrap and paralleling.
Common-mode rejection remains excellent due to the 0.005% laser-trimmed ratio adjustment of internal resistors R1-R4. The impedance matching between ports does not benefit from laser trimming, but by intrinsic matching of the THAT1286 resistors. Although the absolute value of R1-R4 can vary as much as ±30% from lot-to-lot, the internal resistor values of a dual THAT1286 on the same die are typically within 0.5%.
Input current imbalance errors, which are up to 200% in Figure 1 (when the values of R1 and R3 are scaled for -6dB attenuation) are now <1% due to cross-coupled inputs. The circuit can also be built using two single line receivers such as the THAT1246 or INA137 with excellent results. Each input port in the circuit shown in Figure 4 has a nominal 9kΩ input impedance to ground. The differential impedance (between inputs) is 12kΩ.
Cross-coupling using the double-balanced configuration of Figure 4 does not introduce asymmetrical delays in the feedback path. In addition, when balanced outputs are taken from Figure 4, errors from the finite high frequency common mode rejection of the line receivers' internal op amps cancel to improve HF common-mode rejection over that of a single line receiver or the approach in Figure 3.