Handbook for Sound Engineers

Consoles 853

noise and crosstalk characteristics are a good order of

magnitude superior to any analog multitrack recorder,

so this element can be a good choice for an inexpensive

track assignment routing matrix.

25.9.4 Potentiometric Switching

A refinement of this element—in fact, really an exten-

sion of the same principle—is shown in Fig. 25-32C.

Here, a second analog transmission gate replaces the

dropped resistor and is driven through an inverter from

the control line for the original gate, arranging for it to

be on when the other is off and vice versa. When the

original gate is on, there is very little potential across

either of the gates (they’re both at virtual ground from

the op-amp). Similarly, there is little potential across

either of the gates when the second gate is on, since it is

tying the series resistor to ground and the open gate is

between ground and virtual ground. Crosstalk is dramat-

ically improved when the element is off because any

signal present at the series resistor faces the double

attenuation of the series resistor tied to ground by the on

second gate followed by the off original gate into the

virtual-earth input of the op-amp. In the on mode of the

element, there is no input attenuation; hence, there is no

gain and no extra noise contribution from the amplifier.

The only limitation now to the cross-switch leakage

characteristic of this switching element is printed-circuit

card layout and grounding arrangements. Given a good

home, this element is virtually unmeasurable.

It does, however, have one quirk that may preclude

its use in some places. Unless a great deal of care is used

to arrange complementary on-off switching timing for

the two gates, they are both momentarily partially on

together during a switching transition. This, for an

instant, ties the virtual-earth amp input to ground via the

quite low half-on impedances of the two series gates,

creating an instantaneous burst of extremely high gain

from the amp; this shows as a transient of noise or worse

still as a splat if any dc offset is present at the

virtual-earth point. It can be minimized, or at least the

extent of the transient defined, by a small value resistor

(Rss) in series with the input, Fig. 25-32C. This will, of

course, increase the signal voltage across the gates and

increase the distortion, so a compromise has to be struck

to suit the given application. Even so, excessive distor-

tion owing to this has never shown itself to be a problem.

25.9.5 Minimizing Noise

To reduce the thermal noise contribution as part of the

circuit noise performance, the resistances involved in

switching should be as low as practically possible

consistent with device limitations and the ground

current arrangements. The feedback resistor around the

virtual-earth stage is limited by the output drive capa-

bility of the op-amp, bearing in mind it has to drive its

load, too. Fig. 25-31B demonstrates a typical channel

resistance variation of a CMOS switching element with

through current. It behaves linearly until about 40 mA,

which actually compares more than favorably with the

output drive current capability of an op-amp. (FETs are

excellent constant-current sources, self-limiting in

nature.) As a rule of thumb then, the resistors used

around analog gate switching circuits can be as low as

2.2 k: without exceeding device limitations; the

high-output current capability of the 5534 can be used

to good effect here if the drive for lowest possible

thermal noise is that important. Generally, ground-borne

noise generously provides a noise floor well before this

theoretical limit is attained; the whole lowest-imped-

Figure 25-32. Switching arrangements using CMOS trans-
mission gates.

RS

RS

RS

RS

RD

RS RSS

RS

C. With second analog transmission gate

replacing the dropped resistor.

Inverter Smallseries

resistor

B. With dropping to ground an equal-value

resistor to the series resistor.

Extra

dropped

resistor

Series

resistor

Control

A. With signal voltage applied against a

virtual-earth point.

Feedback

resistor

Input

Series

resistor

Control

Output