How the internal clamp in the Op-Amp work?[ICL7650S]

Discussion in 'General Electronics Chat' started by booboo, Apr 30, 2015.

  1. booboo

    Thread Starter Member

    Apr 25, 2015
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    Hi guys
    I bought an ICL7650S to work with it but some parts of it is odd to me. questions:

    1. In the first page of datasheet you can see this "Extremely Wide Common Mode Voltage Range +3.5V to -5V" What is this? What's the difference between this parameter and "Operating Supply Range"?
    2. Should I use "OUTPUT CLAMP" to increase the precision of the Op-Amp? because the datasheet says: "The OUTPUT CLAMP pin allows reduction of the overload recovery time inherent with chopper-stabilized amplifiers". I want to amplify an output voltage that is between 0-9mv (from a thermocouple) to 0-3.3v then I need to a non-inverting amplifier with 366 Gain(please don't tell me to choose another Op-Amp. for example I know that an auto-zero-drift I/O rail to rail is better but I want to use this Op-Amp).
    3. What type of connection is this:[​IMG]
    4. This Op-Amp has an internal oscillator. What's the usage of it?
    Thanks in advance
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    For question #3, shielded cable (coaxial cable) is used for the connections to inverting and non-inverting pins of amplifier. The shielding (outside conductor) should be connected near R3. And the far side of that shielding should also be connected back to near R3 with a plain wire. Seems complicated but that will minimize common mode noise - I assume.
     
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  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The 7650 is NOT a normal opamp. It has a chopper-stabilized architecture for very low offset and low drift performance.

    Q1. Input common-mode range and power supply range are two different things. The power supply range is the allowed operating voltage for the chip. The input common mode range is the voltage range for the inverting and non-inverting signal inputs; signals outside this range will not be amplified correctly.

    Q2. If you are sure that your signals are within the input and output operating ranges for this part, then the clamp probably is not needed.

    Q3. That is the schematic indication of guard rings. A guard ring is a ring of copper around a pin pad on a pc board. The part is so sensitive that stray electrons wandering around can affect the accuracy of the output signal. A guard ring helps to assure that the air around the pin is at the same electrical potential as the pin itself. I know it sounds weird, but that's the deal. The input impedance of this part is so high that bare fiberglas is not a good enough insulator.

    Q4. This is the hard one. This amp has a switching network that periodically disconnects the inputs and output from external signals and connects them internally to ground and a storage capacitor. The basic idea is this: The value of some very low voltage DC signals is so low that the internal errors in the amplifier can override them, making the output useless. However, AC amplifiers for very low voltage signals can be very accurate. So in you part, the input is chopped with switches to turn it into an AC signal. that signal passes through the amp. Then it goes through another type of switching circuit and filter to re-establish the DC value.

    http://en.wikipedia.org/wiki/Chopper_(electronics) - scroll down to the section Chopper Amplifiers

    ak
     
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  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    @booboo
    to add to AnalogKid's comments, Another option (From a TI datasheet) for DIP packages with very high input impedance, you can bend up the two input pins so they do not go through the PCB, then directly wire your input wires to them if coming from a sensor that is not on the board. Air is a much better dielectric (insulator) than the epoxy/glass PCB.
     
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  5. booboo

    Thread Starter Member

    Apr 25, 2015
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    Thanks guys,
    I have a problem. the thermocouple has inductance(because the thermocouple is in series with heater. in fact, the thermocouple is a heater and vice versa ) and my ADC is an 3v3. I want to supply Op-Amp with 9 or 8v(I cannot supply it less than 7v). for measuring the thermocouple I have to turn-off the power of the heater(thermocouple) and then read the voltage but as I said my ADC is 3v3 and I don't know how to protect it of overload? probably the Op-Amp output will go to around 7 or 8v for a moment(because of inductance) and this would be fatal for ADC. any idea, suggestion?
     
  6. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Yes. I would put an analog switch between the thermocouple and the op-amp. Activate the switch only during the times the heater power is off.
     
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  7. booboo

    Thread Starter Member

    Apr 25, 2015
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    Thanks joey
    How about a diode clamp?
     
  8. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Only if you limit the current into the clamp with a series resistor. Oh, and if you exceed the amps common-mode range (or otherwise cause it to go out of closed-loop), there is likely a long recovery time.
     
  9. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    BTW, I know you don't want to change chips, *but*, Linear Tech has a nice LTC2050HV that can run up to 11VDC.
     
  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    They also make a switched-capacitor front end part that you can use to stabilize almost any opamp you like. More work that the fully-integrated parts, but also more flexibility.

    ak
     
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