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TL441: Review of schematic drawing of log amp. When changing the gain on the A2 Input (pin 7) it appears to change the gain out

Jonathan Navor
Expert 7845 points
Part Number: TL441

Hi Guys,

Good day. We need your help to check the  of the TL441 log amp of our customer.

When they adjust the R10 Attenuator it appears to change the gain on both outputs of pins 5 and 11. They assumed that the input on pin 7 would only control the output on pin 5 as well as the input on pin 4 would only change the out put on pin 11. Is there anything changes to the schematic that can be made to eliminate this inter action between the 2 inputs? If they set the input gain correctly on pin 4 that works very well and the pin 12 and pin 9 inputs and out put work very well. It is just the inter-action between the adjust of gain with the R10 attenuator effecting the out put gain on both pin 5 and pin 11.

If you need any information please let us know. We are looking forward to your response. 

Daughter Board Logamp Schematic

Best regards,

Jonathan

  • 0
    TI__Guru* 93105 points

    Hi Jonathan,

    I had to redraw and simplify the circuit diagram so I could better see what the customer is attempting to do with the TL441 log amplifier. They have each of clamping op amp amplifier stages set up so that the input gains are -25 dbB, 0 dB, +25 dB and +50 dB, providing for a wide dynamic input range. It appears to be similar in concept to the TL441 datasheet, Figure 15, a Logarithmic Amplifier with Input Voltage Range Greater than 80 dB. I do think upon review that the customer's implementation is correct.

    I suspect the effect observed that both the pin 5 (Yout) and pin 11 (Zout) are changing when R10 is adjusted could be due to the simplicity of the overall TL441 design, or something about its layout permitting the change in Zout. It would seem that the change in Zout should be much less than what is being observed in Yout. Is that the case?

    If it isn't something about the TL441 design, or layout itself it could have something to do with the output design and load the customer is applying. It might be that the Yout output is talking to the Zout output and giving the appearance that Zout is changing in response to R10 being adjusted. The TL441 output impedance is listed as 200 Ohms typical, which is quite high. The TL441 datasheet shows in Figure 14 and 15 a load resistance of about 4 kilohms off each output. The customer has a configuration of a 1 kilohm resistor in series with each output, then summed together at a 1 kilohm to ground. That certainly is a much heavier load and isn't the same output configuration as applied in the TL441 datasheet circuits. There is a note in Figure 16 stating "R designates resistors of equal value, typically 2 kΩ to 10 kΩ" and I think that should be observed. 

    Additionally, there may be some performance merit had by connecting the TL441 Yout and Zout outputs, to the inputs of a balanced input, difference amplifier as seen in datasheet Fig. 15.

    Regards, Thomas

    Precision Amplifiers Applications Engineering