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RE: VFC320 question

Madhu Mohan
Prodigy 60 points
Other Parts Discussed in Thread: VFC320

Dear Bruce Trump,

 We are using VFC 320 in our application for the range of 0 to 10 V input output of 0 to 100 KHz.

The circuit was calibrated in the room temperature (24°C) and tested for more than one week in the lab.

When the circuit was deployed in the field (Approximately 30 to 40°C), the output started to reduce. Up on investigating, it was found that the output is varying drastically depending on the temperature. It was also noticed that the output is back to normal when tested at room temperature again. The drift was around 100 to 150 Hz

The supporting components used are all high precision and all the components are tested individually for the temperature dependency. During the individual test, it was found that VFC320 is contributing to the drift.

Please let me know how to tackle the issue.

Expecting your reply,

Regards,

Madhu.

  • 0
    Guru 21975 points

    Madhu,

    Can you please provide specifics on the magnitude of the drift?  The change was 100 to 150 Hz at a nominal frequency of 100kHz, is that correct? And the change in temperature was from 24'C to 40'C, is that correct? How did you isolate the drift of individual components? In particular, the one-shot capacitor can be very sensitive. Was the temperature of the VFC320 changed without changing the temperature of the one-shot capacitor? What type of capacitor is used for the one-shot?

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 60 points

     

    Dear Bruce Trump,

    You are correct with the frequency drift of 100 to 150Hz and the temperature of 24 to 40'C.

    All the supporting components like Precision Metal Film resistors are with ± 15ppm/°C and tolerance of ± 0.1% .the one shot capacitor used are 0ppm/°C to +70ppm/°C and tolerance of ± 1% Silvered Mica Capacitors.

    Was the temperature of the VFC320 changed without changing the temperature of the one-shot capacitor? Yes, This also was done by plugging the one shot capacitor at back side of the PCB.

    During the testing, individual components were heated using controlled and pointed gun. the frequency started drifting as soon as the gun was pointed to VFC and later just by blowing air with mouth on the VFC, the frequency returns back. Approximately we have used 4800number of VFC320 till date but the fluctuation was noticed during current deployment.

    Expecting your reply,

     

    Regards,

    Madhu.

     

  • 0 in reply to Madhu Mohan
    Guru 21975 points

    Madhu,

    Thanks for the additional information. It appears that you have done a reasonable job of isolating the drift to the VFC320.

    The specified drift of the VFC320BP version is 50 ppm/'C maximum with a 10kHz full-scale frequency. Figure 3 in the data sheet shows that the typical gain drift increases from approximately 30 ppm/'C to 80 ppm/'C at 100 kHz. So calculating the expected typical drift:   80 ppm/'C * 16 'C = 1280 ppm = 128 Hz at 100 kHz.  So it seems that the drift is in the range that could reasonably be expected. The high performance grade, VFC320CP would likely cut this drift in half.

    External components (primarily the one-shot capacitor) may add (or subtract) from this drift. (I understand that you removed this factor in your tests.) The silver-mica capacitor you used (70 ppm/'C) might degrade this further (or might possibly improve matters, if you are lucky). Note that the polarity of our drift may be uncertain whereas the drift of the silver-mica capacitor may be more consistent.

    Low frequency (10kHz ful-scale) drift of the VFC320 is primarily determined by the internal 7.5V bandgap reference voltage which is laser trimmed for accuracy and drift. As frequency is increased, switching times and other parasitic effects start to play a role.

    From your description, it seems that you have previously received devices that appeared to have lower drift. Drift may vary from lot to lot. The actual drift curve may follow an "S" shape. Our drift is specified over the full specification range, -25 to 85'C. Your narrower frequency range may have operated in a "sweet spot" in temperature where the drift was especially low. It's also possible that its drift was partially cancelled by drift of the one-shot capacitor.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 60 points

    Dear Bruce Trump,

    Thank you for the support,

    Regards,

    Madhu.