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Analog switch configuration

Discussion in 'Electronic Design' started by PPP, Dec 11, 2006.

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  1. PPP

    PPP Guest

    Good afternoon, folks!

    I have a question regarding the use of analog switches. The switch that
    I am using is a Texas Instrument TS5A3159A. The datasheet link is found
    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?

    Thank you!
     
  2. 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



    martin
     
  3. Guest

    You might try using the input switch the other way around, so that the
    1uF/22k gong into the op amps sumiming junction is either connected to
    the sound source you want or connected to ground. At the moment the
    1uF/22k are acting as an antenna whenever the switch isn't connecting
    them to your sound source.

    You should note that there is a certain amount of capacitance between
    the power rails and the digital control inputs and your switched output
    on your digital switch even when the switch is nominally open circuit.
     
  4. niftydog

    niftydog Guest

    Rapid changes in DC offset conditions. Because of the VCC/2 bias
    configuration, and the fact that no capacitor is ideal (leakage), each
    time you switch you get a rapid settling of the DC conditions which
    manifest as a thump or a pop. One way to minimise it would be to use a ±
    supply and/or op amps with offset null.

    But, first I would try putting a DC blocking cap in the output path of
    the op amp. Might help a little.

    nifty
     
  5. tempus fugit

    tempus fugit Guest

    Have you tried biasing the anolog switch inputs and outputs at 1/2 its
    supply voltage? You would then need blocking caps to keep the DC out of the
    audio signal path.

    Try using a voltage divider to get the 1/2 supply voltage and coupling it
    through a 1 meg resistor. Connect it through a separate resistor for each
    com and NO input. You should be able to get a way with 1 connection to the
    NC, since it is a direct connection.
     
  6. martin griffith wrote...
    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 datasheet 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
     
  7. Nice ascii Win

    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


    martin
     
  8. Ban

    Ban Guest

    Since you have already a properly biased input signal from the previous
    stage, the phase inverter can be made this way:
    ___ ___
    o--+-|___|---+--|___|--.
    | 10k | 10k |
    | | |
    | | |\ |
    | '--|-\ |
    | ___ | >---+
    '-|___|--+---|+/
    10k | |/OPA365
    |
    \ o
    \
    \.
    o
    |
    Vcc/2 ---+
    |
    ---+
    ---
    |47u
    ===
    GND
    (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
    I recommend this opamp for single supply R/R non-inverting applications.
     
  9. martin griffith wrote...
    Thanks! I just now corrected a small Vcc/2 typo above.
    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.
     
  10. On 12 Dec 2006 02:47:30 -0800, in sci.electronics.design Winfield Hill

    chomp

    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
     
  11. Jim Thompson

    Jim Thompson Guest

    I've made several synchronous demodulators using that technique... the
    first time would have been around 1970-71.

    ...Jim Thompson
     
  12. jasen

    jasen Guest


    there's a number of issues I can see,
    one is the sudden voltage step the op-amp input will see, the other is the
    similar step produced at the output.
    as others have said audio is ground referenced so you need a two-rail setup.

    if your 3v is two dry cells use the mid-point as earth and power the chips
    from
    +1.5 and -1.5

    if you can reduce loose the first switch and leave the op-amp input
    in-circuit all the time

    as I read it your input ac signal should be referenced from VCC/2 after
    the low-pass filter, but is shown ground referenced, is this an error?

    re-draw the circuit as a dual supply setup and something obvious
    may jump out at you,


    the 1uF capacitor looks kind of high for a filter capacitor, if you're not
    making a high-pass filter there loose it.

    the first switch adds as far as I can see nothing but noise to your system,
    loose it too.

    try this circuit, see if it does what you want,
    the virtual earth of the inverter op-amp
    shouldn't inject any audio distortion into the passthrough channel


    |+1.5V
    ...... phase select input
    nc: <-- (pulse from monostable)
    .-----------------------------o_ :
    input from | : ~o--------- output
    Sallen-Key --+--[2.2k]--+--[2.2K]-----+----o :
    low-pass | | no:....:TSA3159A
    filter | |\|+1.5v | |-1.5V
    `---|-\ |
    | >------'
    0V --------|+/ TLV2784A
    |/| -1,5V

    Basically your analogue ground is 1.5V above the digital ground.

    Using a single-point analogue ground will work better than deriving it
    ad-hoc wherever it's needed, even if you spend an op-amp on providing it.

    If your 3V supply comes in two 1.5V parts tap the mid point for your
    analogue ground. it means you need a two pole switch for the power but
    it'll probably be worth it.

    this is all guesswork, and I'm no expert, also if your input is not low
    impedance and your output load is not a high-ish impedance it'll probably all
    go pear-shaped
     
  13. PPP

    PPP Guest



    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.
     
  14. PPP

    PPP Guest


    Hi Win! Thanks for the input!

    I was actually contemplating using only one switch. But I wondered if
    using only one switch will decrease the switch isolation. I was doing
    some initial simulation on PSpice using a similar circuit above and
    using another analog switch. The results indicated that there were some
    coupling from the NO to the NC terminal. But then again I don't know
    how the real circuit would respond.

    Also, I was thinking that with the configuration above the inverting
    opamp would still be inverting a signal even if its function is not
    required. Wouldn't this be unwanted excess power?

    Thanks!
    P.
     
  15. PPP

    PPP Guest

    I could switch to design to dual supply, but I think working with
    single supply would be a learning experience for me. So, my audio
    signals thru out the circuit are being biased at VCC/2.

    Good point. This is the first time I've dealt with this type of switch.
    The only information I gathered about them is from the application
    notes... namely, from TI and Maxim's. Somewhere there, I read that in
    order to prevent distortion in my audio signal I should use a switch
    with a low on-resistance (Ron).
    So far, I've tried using a MAX4410 and a TPA6102A2. The MAX4410 seems
    very sensitive to its surrounding, and since I am not a master in PCB
    layout, I'm afraid that I may not get it to work properly. The
    TPA6102A2 seems to be less sensitive, but I'm hearing a lot of static
    noise in my headphones even when the sound is off. However, I think
    this may be caused by the noisy output of my soundcard because when I
    took the sound input off from the circuit it became silent.


    Thank you!
     
  16. PPP

    PPP Guest

    I tried this and unfortunately, I did not notice any difference. If
    this did work, how does the 100K actually eliminate the click?

    I finally was able to probe the output and noticed something
    interesting...well, to me at least. I decided to connect the output of
    the second switch (right hand in the pic) to a capacitor (~ 2uF since I
    didn't have any other values). I just wanted to see the output on the
    scope and was hoping to eliminate the DC bias of the audio signal.
    However, when the switch switches, the signal bias shifts from 0V to
    300mV. Not sure why.

    Thank you!
     
  17. jasen

    jasen Guest

    I was actually contemplating using only one switch. But I wondered if
    what effect would that crosstalk have in this circuit?
    there may be some tiny reduction in amplitude, but would it do anything else?
    yes, but if you switch the input to the inveryer off it'll bounce round a
    bit when you switch it back on.

    using larger resistances will reduce the power needs but may increase
    distortion,

    if you can arrange to switch the front switch on before the back one and off
    after it that could reduce power consumption.

    how much power does the switch need to operate?

    conversely there may be a way to confiure your next stage to invert or not
    and reduce this stage to a single analogue switch. which just reconfigures the
    next stage....

    Bye.
    Jasen
     
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