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VFC110: AP

Decheng Yan
Prodigy 20 points
Part Number: VFC110
Other Parts Discussed in Thread: VFC320, VFC32

Hello,

I have a question regarding to VFC 110AP that is produced by Texas Instrument. In my research experiment, I've been using this for voltage-frequency sensor. So, I connect the input to a voltage source and count the output voltage frequency. The voltage-frequency relationship will be according to the datasheet a linear function:

I_input * T = I_ref*TO. 

which is equivalent to I_input = I_ref * TO * F_OUT. Now my question is how large is the one-shot period TO? I just want to make sure that TO is a constant and I can repeat the experiment and get the same result every time even if I use another VFC 110 sensor. Or does every sensor have a different TO value?

Thank you for giving me an exact description of TO if you have any! Since I'm gonna publish a paper using this in a very short time, could you please answer this question asap?

With best regards,

Decheng

  • 0
    TI__Guru* 93105 points

    Hello Decheng,

    The VFC110 is a legacy product from the late 1980s and we longer have any technical information about its internal design spec's. The datasheet is the only information we have to work with. 

    The To should be reasonably consistent between devices. Probably the best resource we have in the VFC110 datasheet regarding To is the PRINCIPLE OF OPERATION sidebar on Page 8. Using the information in that sidebar we should be able to get some of the To timing:

    IIN = IREF x Duty Cycle

    VIN / RIN =  IREF x Duty Cycle

    VIN / RIN = IREF x fOUT x To

    To = VIN / (RIN x IREF x fOUT)

    Reading the various other VFC32 and VFC320 datasheets To is typically set to about 25 % 1/fOUT.

    Let VIN = 10 V, and let RIN = 40 kilohm per Pg. 4, FULL−SCALE FREQUENCY vs EXTERNAL ONE−SHOT CAPACITOR graph

    It states in the PRINCIPLE OF OPERATION text that IREF is approximately 1 mA.

    If fOUT is set to 100 kHz, then:

    To = 10  V / (4e4 x 1 e-3 x 1e5) = 2.5 e-6 = 2.5 us

    and certainly that is 25 % of 1/100 kHz = 10 us.

    The VFC110 datasheet doesn't provide a formula for determining the Cos (one shot) capacitor value. It has the one FULL−SCALE FREQUENCY vs EXTERNAL ONE−SHOT CAPACITOR graph based on RIN equal to 40 kilohm. All of the rest of the Cos information is in the Component Selection Table 1, on Pg. 7. It is stated there "The values given were determined empirically to give the optimal performance, taking into consideration tradeoffs between linearity and jitter for each given full-scale frequency of operation." The actually duty cycle looks to be different and optimized for the various FULL-SCALE FREQUENCY, fFS listed.

    Regards, Thomas

    Precision Amplifiers Applications Engineering