A Simple AC-Coupled Balanced Line Reciever

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A Simple AC-Coupled Balanced Line Reciever

Post by mediatechnology » Fri Nov 16, 2012 5:22 pm

I needed an AC-coupled, high impedance balanced line receiver with exceptional low frequency common mode rejection.

I wanted to use a bipolar input op amp with low-value film capacitors for a "non-electrolytic" approach.
This requires a high impedance input to reduce capacitor matching requirements or bootstrapping a relatively low impedance to make it look larger.

One of the concerns having large value bias resistors are noise, due to input noise current and offset due to high bias current.
Whitlock's InGenius papers make the case for low value bias resistors bootstrapped to look like high value ones: http://www.jensen-transformers.com/an/ingenaes.pdf
Then I remembered T-bias and what it provides Mic inputs: viewtopic.php?f=6&t=191&p=2404

AC-Coupled Line Input with T-Bias Network to Raise Input Common Mode Impedance

A high differential input impedance is not required. The LF cutoff for differential signals is <8 Hz.
R1 and R2 provide differential termination.
R3 provides a DC return for op amp bias currents, both DC and noise, as well as a bleed path for the capacitors. (Bleeders on the LH side are not shown.)

Normally huge DC offsets will develop across a 1M R3 and they do here.
The input noise currents also convert to a relatively large voltage across R3.
(Do note that the source impedance in series with the reactance of Cin are also in parallel with R1+R3.)

What makes the circuit practical is that these current-induced voltages appear in common mode.

Common mode rejection is realized by the THAT1240. (I needed unity gain otherwise a 1246 would have been used.)
Thus, the noise current developed across R3 is canceled and the DC offset, also in common mode, is virtually eliminated.

Because R3 is so large in value and the common mode impedance high, capacitor matching requirements are greatly relaxed.
This circuit provided -86 dB common mode rejection using 10% film caps regardless of frequency from 10 Hz - 1 kHz and beyond.
There is not any measurable noise or output offset penalty with R3=0 Ohms or R3 = 1M Ohm.

I developed this circuit for the DC-coupled preamp where the "-" input is tied to the floating Vcm supply and the "+" input to the 1510 output.
I had previously used a single-ended BIFET amp and decided that additional common mode rejection was needed.
An added bonus is that a bipolar op amp could be used along with film capacitors.

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