AD8436 DIP Package Output Unreliable

I am attempting to use the AD8436 DC coupled so that the Vout signal is the residual sum of squares (RSS) value of both the AC and DC voltages as seen on page 4 of the the application note here: http://www.analog.com/media/en/technical-documentation/application-notes/AN-1341.pdf

 

I have the AD8436 connected using a single supply (Vcc = 5.0 V Vee = GND) and have connected the chip exactly as Figure 5 (page 3) in the same application note listed above has said to. I am not using the sum pin and only connecting my input to the RMS pin. I am also using the DIP Package on a breadboard for testing purposes.

 

When I ground the input I get 0.237 V on the Out pin. One volt DC voltage applied to the RMS pin results in 0.24 V on the Out pin, and three volts DC results in 1.068 V on the Vout pin. It seems as though I have not configured the device correctly as I am not receiving the unaltered DC voltage on the output pin Vout. I am using a 10uF capacitor for CAVG, and a 0.1 uF for CCF. I have also tried using the SUM pin as the only input and tried using both the SUM and RMS pins tied together as the input, but with no better results.

 

Any help resolving this issue would be greatly appreciated.

  • Hi John,

    Check out the sub-section devoted to single-supply configurations on page 10. Referring to figure 18, notice that the IGND should not be grounded. This is because IGND is actually the common-mode reference for the rms core, or Vs/2. For a 5V-to-gnd supply, this is nominally 2.5V.

    If you decide you need OBUF, or if you use 'OUT', the dc output will reference to gnd, not Vs/2.

    I hope this helps.

    js


  • Thanks for your reply,

    I removed ground from the IGND pin and still do not see the correct DC voltage on the output of the OUT pin. Can you recommend any other changes that might help solve the issue? Also, in Figure 18 CCF is a little translucent, is it not needed in this configuration?

    I will also try a new device to see if I have it configured correctly but have burned the chip up on accident.

  • I'm not sure if this EZ contact is still active but there is another factor I failed to mention. The 'SUM' pin requires a V-to-I resistor. The 'SUM' connotation was used to signify an internal port where one could connect a couple of input resistors to 'sum' inputs, or a place to rescale an input slightly larger than the maximum voltage useable voltage for the built-in 8k resistor. Otherwise one would overdrive the input direct circuitry.

    If you need an ac + dc summation you can just apply the common signal to the input, or introduce two separate signals to the inputs but the 'SUM' pin still requires a resistor with value around 8K.

    I hope this helps,

    js

  • 0
    •  Analog Employees 
    on Aug 2, 2018 2:57 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin