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Op Amp to BS2 Blues — Parallax Forums

Op Amp to BS2 Blues

ArchiverArchiver Posts: 46,084
edited 2004-03-29 15:13 in General Discussion
I constructed the amplifier circuit shown in N&V column #57 to
connect to an ADC as described, except for changing the gain resistor
to 202 ohms (to provide a gain of 100). After trouble shooting the
circuit a number of times, I still can't get any amplification of the
sensor output. The OP amp output measures about 0.3 volts and does
not change with changing input mV. I measured the sensor and can
read changing mV with increasing presssure so I feel like the sensor
works correctly. Is the circuit correct as shown? Does anyone have
any ideas on checking the op amp ICs or how I can split the circuit
and troubleshoot each IC? Any help would greatly appreciated.
Walt Beckwith

Comments

  • ArchiverArchiver Posts: 46,084
    edited 2004-03-26 03:01
    --- In basicstamps@yahoogroups.com, "waltb27707" <wbeckwith@m...>
    wrote:
    > I constructed the amplifier circuit shown in N&V column #57 to
    > connect to an ADC as described, except for changing the gain
    resistor
    > to 202 ohms (to provide a gain of 100). After trouble shooting the
    > circuit a number of times, I still can't get any amplification of
    the
    > sensor output. The OP amp output measures about 0.3 volts and does
    > not change with changing input mV. I measured the sensor and can
    > read changing mV with increasing presssure so I feel like the
    sensor
    > works correctly. Is the circuit correct as shown? Does anyone
    have
    > any ideas on checking the op amp ICs or how I can split the
    circuit
    > and troubleshoot each IC? Any help would greatly appreciated.
    > Walt Beckwith


    Hi Walt,

    Can you post a layout of your unit ?

    Are you breadboarding it ? maybe a pic ?

    Dave
  • ArchiverArchiver Posts: 46,084
    edited 2004-03-26 12:57
    Quite often, If the parts are good, this problem can be traced to not having
    the input drive voltage source ground at the same potential as the device
    (op amp) ground. This is easy to overlook.
    jim
    http://www.geocities.com/jimforkin2003/


    Original Message
    From: Dave Mucha [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=27mInqKEgxNp48bS6GyT6iQ4uhSosDbSBHf52OBFK8jDDRFi6GW_g_xG8PXedkDF-avwJDQu97N3TA]davemucha@j...[/url
    Sent: Thursday, March 25, 2004 10:01 PM
    To: basicstamps@yahoogroups.com
    Subject: [noparse][[/noparse]basicstamps] Re: Op Amp to BS2 Blues


    --- In basicstamps@yahoogroups.com, "waltb27707" <wbeckwith@m...>
    wrote:
    > I constructed the amplifier circuit shown in N&V column #57 to
    > connect to an ADC as described, except for changing the gain
    resistor
    > to 202 ohms (to provide a gain of 100). After trouble shooting the
    > circuit a number of times, I still can't get any amplification of
    the
    > sensor output. The OP amp output measures about 0.3 volts and does
    > not change with changing input mV. I measured the sensor and can
    > read changing mV with increasing presssure so I feel like the
    sensor
    > works correctly. Is the circuit correct as shown? Does anyone
    have
    > any ideas on checking the op amp ICs or how I can split the
    circuit
    > and troubleshoot each IC? Any help would greatly appreciated.
    > Walt Beckwith


    Hi Walt,

    Can you post a layout of your unit ?

    Are you breadboarding it ? maybe a pic ?

    Dave



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  • ArchiverArchiver Posts: 46,084
    edited 2004-03-26 14:58
    Walt,

    202 ohms seems a little low to provide a gain of 100X. Try a pot of 100k and
    see what happens. What is the value of the input resistor? I *think* that
    the gain resistor / input resistor = gain.

    Jonathan

    www.madlabs.info





    Original Message
    From: "waltb27707" <wbeckwith@m...>
    To: <basicstamps@yahoogroups.com>
    Sent: Thursday, March 25, 2004 6:58 PM
    Subject: [noparse][[/noparse]basicstamps] Op Amp to BS2 Blues


    > I constructed the amplifier circuit shown in N&V column #57 to
    > connect to an ADC as described, except for changing the gain resistor
    > to 202 ohms (to provide a gain of 100). After trouble shooting the
    > circuit a number of times, I still can't get any amplification of the
    > sensor output. The OP amp output measures about 0.3 volts and does
    > not change with changing input mV. I measured the sensor and can
    > read changing mV with increasing presssure so I feel like the sensor
    > works correctly. Is the circuit correct as shown? Does anyone have
    > any ideas on checking the op amp ICs or how I can split the circuit
    > and troubleshoot each IC? Any help would greatly appreciated.
    > Walt Beckwith
    >
    >
    >
    > To UNSUBSCRIBE, just send mail to:
    > basicstamps-unsubscribe@yahoogroups.com
    > from the same email address that you subscribed. Text in the Subject and
    Body of the message will be ignored.
    >
    > Yahoo! Groups Links
    >
    >
    >
    >
    >
    >
  • ArchiverArchiver Posts: 46,084
    edited 2004-03-29 04:12
    Dave, Jim, and Jonathan,
    Thank you for your replies. I hard wired the circuit on ONE PAS
    board and put the two OPA 2340 ICs in sockets. I don't have a
    photo. The circuit is at:
    http://www.parallax.com/dl/docs/cols/nv/vol2/col/nv57.pdf Figure
    57.2

    The gain is calculated by: Vout=(Vhigh-Vlow) (1+2R1/Rgain) R1 is 10K
    ohms. The circuit was wired up with 0.22uF bypass caps wired between
    pins 4 (-) and 8 (+) on each OPA 2340. I am powering the circuit with
    12 volts (8 AA batteries) 12 volts energizes the sensor input
    leads. I have a 7805 regulator that drops the voltage to 4.89 volts
    which powers the op amps. There is a common ground between 12 volt
    and 4.89 volt side. The V low sensor output is not grounded. I'm
    not sure how to check ground potentials. I get 4.89 volts from any
    regulated positive to any grounded pin or wire with a Radio Shack
    DMM.

    According to the Spec sheet the OPA2340 can be run from a single
    supply of 2.5 to 5.5 volts. Which is what I did. There is nothing
    about running from a split supply. I'm beginning to think another amp
    might be a better choice.

    Thanks for the ideas Guys. I will keep checking.

    >>>>>>>>>>>>>
    I constructed the amplifier circuit shown in N&V column #57 to
    > connect to an ADC as described, except for changing the gain
    resistor
    > to 202 ohms (to provide a gain of 100). After trouble shooting the
    > circuit a number of times, I still can't get any amplification of
    the
    > sensor output. The OP amp output measures about 0.3 volts and does
    > not change with changing input mV. I measured the sensor and can
    > read changing mV with increasing presssure so I feel like the
    sensor
    > works correctly. Is the circuit correct as shown? Does anyone
    have
    > any ideas on checking the op amp ICs or how I can split the
    circuit
    > and troubleshoot each IC? Any help would greatly appreciated.
    > Walt Beckwith

    --- Walter Beckwith
    --- wbeckwith@m...
    --- EarthLink: The #1 provider of the Real Internet.
  • ArchiverArchiver Posts: 46,084
    edited 2004-03-29 15:13
    I suspect you are saturating your circuit. Measure the voltages you are
    providing from the sensors to the op amps, add the gain for the difference
    between the two, and I suspect you're out of range especially since the op
    amp is running from 5 volts. You are dealing with a gain of about 50 so .1
    volts on the input would put you at the rail on the output.
    jim
    http://www.geocities.com/jimforkin2003/


    Original Message
    From: waltb27707 [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=W2480LznMIeZ9fUZ1K8aIuarJC6NLQB3neYYNs7dBOgckviQT6E7mredKeHRzeHpThORL73XxSDeLa2fM39xP-Pf]wbeckwith@m...[/url
    Sent: Sunday, March 28, 2004 10:12 PM
    To: basicstamps@yahoogroups.com
    Subject: [noparse][[/noparse]basicstamps] Op Amp to BS2 Blues


    Dave, Jim, and Jonathan,
    Thank you for your replies. I hard wired the circuit on ONE PAS
    board and put the two OPA 2340 ICs in sockets. I don't have a
    photo. The circuit is at:
    http://www.parallax.com/dl/docs/cols/nv/vol2/col/nv57.pdf Figure
    57.2

    The gain is calculated by: Vout=(Vhigh-Vlow) (1+2R1/Rgain) R1 is 10K
    ohms. The circuit was wired up with 0.22uF bypass caps wired between
    pins 4 (-) and 8 (+) on each OPA 2340. I am powering the circuit with
    12 volts (8 AA batteries) 12 volts energizes the sensor input
    leads. I have a 7805 regulator that drops the voltage to 4.89 volts
    which powers the op amps. There is a common ground between 12 volt
    and 4.89 volt side. The V low sensor output is not grounded. I'm
    not sure how to check ground potentials. I get 4.89 volts from any
    regulated positive to any grounded pin or wire with a Radio Shack
    DMM.

    According to the Spec sheet the OPA2340 can be run from a single
    supply of 2.5 to 5.5 volts. Which is what I did. There is nothing
    about running from a split supply. I'm beginning to think another amp
    might be a better choice.

    Thanks for the ideas Guys. I will keep checking.

    >>>>>>>>>>>>>
    I constructed the amplifier circuit shown in N&V column #57 to
    > connect to an ADC as described, except for changing the gain
    resistor
    > to 202 ohms (to provide a gain of 100). After trouble shooting the
    > circuit a number of times, I still can't get any amplification of
    the
    > sensor output. The OP amp output measures about 0.3 volts and does
    > not change with changing input mV. I measured the sensor and can
    > read changing mV with increasing presssure so I feel like the
    sensor
    > works correctly. Is the circuit correct as shown? Does anyone
    have
    > any ideas on checking the op amp ICs or how I can split the
    circuit
    > and troubleshoot each IC? Any help would greatly appreciated.
    > Walt Beckwith

    --- Walter Beckwith
    --- wbeckwith@m...
    --- EarthLink: The #1 provider of the Real Internet.




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