Re: Analog switch configuration

Winfield Hill wrote:
martin griffith wrote...

"PPP" pispaspos wrote:

I have a question regarding the use of analog switches. The switch
that I am using is a Texas Instrument TS5A3159A. The data*** link
is here: http://focus.ti.com/docs/prod/folders/print/ts5a3159a.html

This is my schematic:
http://img361.imageshack.us/my.php?image=switchschematicoo4.jpg

If I disconnect the inverting opamp and just directly connect the NO
channels together, I don't notice any popping or clicking noise in my
audio output. But when I insert the inverting op amp, the popping and
clicking becomes apparent.

What causes the clicking / popping noise in this configuration?
Is there another way to configure this circuit?

Try putting a 100K to gnd before the 1uF
try a 22uf in series of the o/p of the opamp, then a 100K to gnd

Also put a scope on the o/p of the opamp see if there is anything
funny happening.

Exactly right. Here's pispaspos' circuit.

. o--------------------------o
. ---o / \ o----- out
. o--||---R--+----R-----+----o
. | _ | | invert
. '--|- \ | 100k
. | >---' |
. Vcc/2 ---|+_/ gnd

When he selects the invert position, the signal has Vcc/2 added
to it. When he selects the direct position, it doesn't. Hence
the click. Your suggestions solves that problem by eliminating
the dc signal from the inverted pathway.

. o-----------------------------------o ts5a3159
. ---o / 1uF 2.2k 2.2k 22uF \ o----- out
. o--+--||----R--+----R-----+--||--+--o
. | | _ | | invert
. 100k '--|- \ | 100k
. | | >---' |
. gnd Vcc/2 ---|+_/ gnd

This still leaves the problem that pispaspos is switching a signal
at ground level, with a single-polarity-powered CMOS switch that
can only operate to -0.6 volts below ground, without clipping.

On the data*** page 3, we find "Absolute Minimum and Maximum
Ratings" that say, "Analog voltage range min -0.5V, max V+ +0.5V,
which means that all the switched signals must be no more than
0.5V below ground, or 0.5V above the supply rail. In the absence
of a coupling capacitor, the renegade HV capacitors can be charged
appropriately, but this can result in biat-setpoint clipping and
distortion.

We can solve this by placing the 22uF coupling caps on the
in and out signal lines, outside of the switches:

. 22uF o---------------------o 22uF
. --||-+--o / 22k 22k \ o--||--+----- out
. | o---R--+----R-----+---o | headphone amp
. 10k | _ | invert |
. | '--|- \ | 10k
. | | >---' |
. +----------------|+_/ gnd
. |
. Vcc/2

Or, dealing with the Vcc/2 bias issue head on, assuming the
input-signal dc path is present and accounted for.

. 22uF o---------------------o 22uF
. --||-+--o / 22k 22k \ o--||--+----- out
. | o---R--+----R-----+---o | headphone amp
. 10k | _ | invert |
. | '--|- \ | 10k
. | | >---' |
. '---+--10k---+---|+_/ gnd
. Vcc/2 | _|_
. ---Rx---+ --- 10uF
. 2.2k | |
. Rx gnd
. 2.2k
. |
. gnd


--
Thanks,
- Win



Thanks for the input everybody! I'm still digesting the response.
However, just a quick comment. My audio source is actually biased
around VCC/2. The single supply low pass filter connected to the COM
terminal of the first switch outputs the audio signal "riding" on
VCC/2.

I am actually trying to build a voice over circuit. I thought the
easiest way to do this was by using switches. The inverting circuit in
my diagram is there so I can tell if my circuit was really switched.
Hopefully, once I get things figured out, I will replace it with an
attenuator.

.