Re: Analog switch configuration
- From: martin griffith <mart_in_medina@xxxxxxxx>
- Date: Tue, 12 Dec 2006 14:37:21 +0100
On 12 Dec 2006 02:47:30 -0800, in sci.electronics.design Winfield Hill
<Winfield_member@xxxxxxxxxxx> wrote:
chomp
Nice ascii Win
Thanks! I just now corrected a small Vcc/2 typo above.
On second thoughts, get rid of the first switch, connect the 1uf
to the signal in, keep the 22uF o/p cap. I suggested. So the
o/p switch just switches between the input and the inverted
signal.
Excellent idea to eliminate the input switch.
. -----+--------------------------------o
. | 1uF 22k 22k 22uF \ o----- out
. '--||-----R--+----R-----+-||--+--o headphone amp
. | _ | | invert
. '--|- \ | 10k
. | >---' |
. Vcc ---10k---+---|+_/ gnd
. 4V max | tlv2780
. 10k
. |
. gnd
But using only one large 22uF cap means the remaining switch is
still limited to a -0.5V swing, potentially causing clipping, or
if the source signal is ac coupled, causing capacitor charging
on the negative peaks. An extra electrolytic would eliminate
any issue during full 3V peak-peak signal swings.
. 47uF ts5a3159
. --||-+----------------------o 47uF
. | 22k 22k \ o--||--+----- out
. '----R--+----R-----+---o | headphone amp
. | _ | invert |
. '--|- \ | 10k
. | >---' |
. Vcc ---Rx--+---|+_/ gnd
. 3.0V | tlv2780
. 4V max Rx
. |
. gnd
On the other hand, we don't know about pispaspos' source and
destination circuit biasing, perhaps the electrolytics can be
eliminated in the full design, after the dust settles.
Now, addressing pispaspos' choice of cmos switch IC. Why use a
large-area 1.3-ohm switch for relatively high-Z audio signals?
The coupling-cap electrolytic sizes, large as they are, aren't
meant for use with loads much below say 1k, which implies a 5,
10 or even 20-ohm switch should be fine. To my mind, if the
switch was dealing directly with a 40-ohm headphone signal, then
a 1-ohm Ron would be well used. I know the ts5a3159 is cheap,
only 56 cents, but somehow it just seems like overkill to me.
I wonder what pispaspos will choose for his headphone amplifier
in his 3-volt low-voltage powered system?
Commenting on TI's tlv2780 amplifier series, which pispaspos
selected, http://focus.ti.com/docs/prod/folders/print/tlv2780.html
it's pretty ballsie for TI to bring out a full family of opamps
(six types, in four packages, including a legacy miniDIP) with
only a 4-volt maximum supply-voltage rating! Whoa! Couldn't
they push it up to say 5.5V, to expand the market? Also, these
opamps have pretty wimpy output transistors for a low-voltage
cmos process: 33 ohms for the p-type when powered at 2.7 volts,
and 50 ohms at 1.8 volts. This means you need to limit your load
draw to say 2mA (or 1k) to avoid losing too much of your already-
limited low-voltage output swing. Sheesh, couldn't TI have done
better with their fabulous 4-volt cmos process? I mean, look at
their 1-ohm switch (5-ohms at 1.8 volts) in a 6.5-volt process.
3v3 rail and audio = a horrible mess
I still haven't come to terms with audio ADCs etc with a single 5V
rail, and will stick to a minimum supply of +-12V until they prove the
world isn't flat.
Ban's idea ( well I saw it first in Walts' audio opamp cookbook) is
rather nice, But how about an audio transformer, with the o/p CT to
ground and the cmos switch flipping between the in and out of phase
terminals
martin
.
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