Can ADL5304 be used without internal bias on pin 4, still function as a Log-Amp?

Hello,

I'm trying to use  ADL5304 as a log-amp without having bias on pin 4, I need to measure 10pA-3mA!

Thank you.
Shawn

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  • 0
    •  Analog Employees 
    on Aug 25, 2020 11:01 PM 3 months ago

    Hi Shawn,

    I'm not exactly sure what you mean here. If you want to use the device at zero Volts input, the answer is YES, see datasheet section on Using a Negative Supply. With negative supply, now VSMx pins can be tied to ground. An example is shown in Figure 50. Output voltage will be proportional to log(InputCurrent). With a series resistor added, output voltage will be proportional to log(InputVoltage).

    Note that log converters such as ADL5304 ALWAYS need some positive input current. Remember, the log of zero or a negative number is undefined. If that presents a problem, your application needs to externally inject a small amount of leakage current into the input pin, to keep input always positive current. That small leakage current error can be subtracted in floating point math externally, after making the measurement with the ADC.

    Hope that helps answer your question.  -Bruce H.

  • Hi Bruce,

    That is exactly what I was looking for but in a single supply, is it possible to measure 1pA with dual supply?

    Thank you for your help.

    Best regards,
    Shawn

  • What we really need is Figure 50 configuration in a single supply, I understand that it may not be possible, will figure 44 apply to dual supply? it shows 1pA to 10mA! from -1 to +1 volt output.

    Thank you.

    Best 
    Shawn

  • +1
    •  Analog Employees 
    on Sep 1, 2020 11:40 PM 2 months ago in reply to smoazzeny

    Hi Shawn,

    Yes, Figure 44 does apply. The VLOG output does shift downward by the same amount of the reduction in VSMx voltage. Sorry I didn't mention that earlier.

    -Bruce H

  • Good day Bruce,

    Thank you for your reply and confirmation, as you have noticed I'm not an expert on this device but thanks to you I'm learning fast. is it normal to connect a capacitor from pin 31/32 to pin 4 to stop the oscillation? .01uF is the minimum to stop the oscillation but it takes up to 10 to 20 seconds for the output to stabilize! we switch the input using a mux to measure different sources! 

    Any suggestions to reduce the settling time and stop the oscillation? I have used all the suggestions in the data sheet but this method is the only one that appears to work! I have .01uF on pins 31 and pin 10 to ground, pins 31/32 are shorted as well as pins 9 and 10. using resistor cap combinations on these pins make the oscillation worse.

    Thank you.
    Shawn

  • 0
    •  Analog Employees 
    on Sep 5, 2020 1:16 AM 2 months ago in reply to smoazzeny

    Hi Shawn,

    No, it is not normal to need feedback capacitor as you've described to stop any oscillation.

    The problem might be too much capacitance from input pin INUM to ground. Is there long wire(s) connected? Could there be a lot of stray capacitance to ground? If so, try a series resistor nearby to the INUM pin 4. Use just the minimum amount of resistance, so that the voltage drop up at the highest input current doesn't cause problems.

    Remember internal to the IC, the output of the JFET-input op-amp feeds back to the input through the logging transistor, so loop gain can be very high. Input capacitance adds extra phase shift to that path, which means a delayed feedback, which could be the root cause of the oscillation you describe. External resistance (or a large inductance) in series with INUM pin reduces the feedback phase shift.

    -Bruce H

  • Hi Bruce,

    Thank you again for the tips and suggestions, I have a question regarding dual supply operation with figure 44 configuration, is it okay to use ground as shielding throughout the circuit including around pin 4 input?

    BTW, I tried using 100 ohms through 2.5k resistor on the input, no improvement, what would be your suggestion on the value of an inductor to start? we do have hi capacitive load with long lines on the input!

    Best regards,
    Shawn

Reply
  • Hi Bruce,

    Thank you again for the tips and suggestions, I have a question regarding dual supply operation with figure 44 configuration, is it okay to use ground as shielding throughout the circuit including around pin 4 input?

    BTW, I tried using 100 ohms through 2.5k resistor on the input, no improvement, what would be your suggestion on the value of an inductor to start? we do have hi capacitive load with long lines on the input!

    Best regards,
    Shawn

Children
  • 0
    •  Analog Employees 
    on Sep 8, 2020 3:38 PM 2 months ago in reply to smoazzeny

    Hi Shawn,

    1. With VSMx at ground, in dual-supply configuration, the voltage at INUM will normally servo to ground potential also, so a zero volt guard is fine.

    2. Unless you observed 2500 Ohms showing some improvement, it's doubtful that an inductor would help.

    To help isolate the problem, you might try completely disconnecting the long input lines right near the ADL5304. Instead, supply a positive input current derived simply from a resistor between INUM and either 2VLT or VPOS. Keep the resistor lead connecting to INUM short or shielded (or both), to minimize noise pickup. With external influence from long wires removed, verify that VLOG is relatively free of noise and oscillation and the correct DC value. Then try a few other resistor values that span a large range of input current, again confirming that without external influence of long wires, the circuit is stable as-built on the PCB. If instability persists, this way we know the problem is on the PC board somewhere. If problem goes away, we know to look closer at the long interconnecting wires arrangement.