SSM2017

–8–

REV. C

BUS SUMMING AMPLIFIER

In addition to is use as a microphone preamplifier, the SSM2017

can be used as a very low noise summing amplifier. Such a cir-

cuit is particularly useful when many medium impedance out-

puts are summed together to produce a high effective noise gain.

The principle of the summing amplifier is to ground the SSM2017

inputs. Under these conditions, Pins 1 and 8 are ac virtual

grounds sitting about 0.55 V below ground.

To remove the 0.55 V offset, the circuit of Figure 24 is

recommended.

This places Pins l and 8 at a true dc virtual ground. R4 in con-

about any operational amplifier will work well here since it is re-

moved from the signal path. If the dc offset at Pins l and 8 is not

too critical, then the servo loop can be replaced by the diode bi-

asing scheme of Figure 24. If ac coupling is used throughout,

then Pins 2 and 3 may be directly grounded.

Figure 24. Bus Summing Amplifier

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

8-Pin Plastic DIP (P) Package

8-Pin Hermetic DIP (Z) Package

16-Pin SOIC (S) Package