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OPA4134: Problems with reading a digital push button POT

ALbert Tello
Intellectual 735 points
Part Number: OPA4134
Channel outout is Pin 14 4.2V , VCC=5V, Vin+= 1.3 Vpk , Vin- R1= 3.9K, R2 =6.8K
Provide case details or comments
The input + Pin 12 = sin Vpk-pk = 1.7 V, ., thias isgnal cames from cell A from the same OP We use Av = R2/R1 +1 ; 6.8/K/3.9K +1 , ' The problem pin 14 is always at 4.2 v V+ = 5V V- = GND What can be the problem Regards Albert Tello

I am using this part with push-button Digital POT 80K in the Neg Input is signal. and the DIG POT is the feedback resistor, AV = - (DIG POT = 80K) / 3.9K why I can not see amplification with the scope

  • 0
    TI__Guru* 93105 points

    Hello Alberto,

    It looks from your circuit description that you are powering the OPA4134 from a single, +5 V supply (pin 4 at +5 V, pin 11 at 0 V). The OPA4134 datasheet indicated the minimum supply voltage is +/-2.5 V, but when one attempts to use it in this manner the operating range is extremely limited. For example consider the op amp's Common-Mode Voltage Range (Vcm) which is specified as [(V–)+2.5] to [(V+)–2.5 V]. If you plug in 0 V for V- and +5 V for V+ the Vcm range for your circuit is +2.5 V to +2.5 V which indicates that about all that could be applied to the non-inverting input is a +2.5 V dc level. You state you are applying a 1.7 Vpp signal to pin 12 which is is a Vcm voltage range of 1.7 Vpp that violates the specification. The op amp will not function correctly when operating in this condition.

    The OPA4134 really needs to be powered with a higher supply voltage, and ideally a dual supply such as VS = ±15 V. At an absolute minimum the supplies need to be able to accommodate the 1.7 Vpp, or  ±0.85 V signal and the Vcm limits. If you take 0.85 V + 2.5 V the total is 3.35 V and for that case the minimum supplies would need to be at least ±3.35 V. However, that is not the only limit that must be considered. The output voltage swing must also be considered.

    The datasheet lists the Voltage Output limit as [(V–) +1.2] to [(V+) –1.5 V] for a load resistance  RL = 2 kΩ. If the input voltage is ±0.85 V and the gain is G = 1 + (6.8 k/ 3.9 k) = 2.74 V/V, the output will be ±0.85 V (2.74) = ±2.33 V. Taking the most limited case of the two the positive swing limit, it would be +2.33 V + 1.5 V, or +3.83 V would be the minimum supply. To be safe, the OPA4134 should in this case I would recommend a supply of at least ±5 V. 

    The OPA4134 could be powered by a single supply such as the +10 V, but then the Vcm and output range limits have to be considered and observed with respect to that supply level. 

    I suggest you have a look at the first few sections of the TI Precision Labs - Op amps series. They nicely explain the Vcm and Vout range concepts I am describing.

    https://training.ti.com/ti-precision-labs-op-amps

    Regards, Thomas

    Precision Amplifiers Applications Engineering