From: firstname.lastname@example.org (Bill Sloman)
Subject: Re: frequency to voltage converter
Date: 17 Jan 2003 08:58:29 -0800
References: <email@example.com> <firstname.lastname@example.org>
NNTP-Posting-Date: 17 Jan 2003 16:58:29 GMT
Winfield Hill wrote in message news:...
> Meindert Sprang wrote...
> > A different approach was published many years ago in Elektor.
> > They used a single CMOS flip-flop, triggered by the signal to
> > be measured. In series with the VSS pin and power ground was
> > a resistor and capacitor in parallel. They claimed that the
> > current draw of the FF was linear with the operating frequency,
> > thus creating a frequency dependent voltage over the resistor.
> . +5 Vcc Cute Frequency to Voltage Converter
> . ---+---------------->
> . +----||----, dV
> . | R +----->
> . '---/\/\---+ Vdd
> . __|__
> . ,--|D Q| Vdd = Vcc - f R C Vdd/2
> . | | |
> . f-in ---|--|> | dV = f R C Vcc/2
> . | |___/Q|--, for very small dV
> . |___________|
> Cute. This is based on the classic formula i = C V f, where
> C is a logic circuit's switched capacitance. That's the amount
> of circuit capacitance that must be charged and discharged each
> cycle. Charge is taken from the supply, q = C V, and shunted
> to ground during discharge, completing the current path. The
> total current is I = q f, hence the formula above (the reader
> is invited to consider the 1/2 term in the output equation).
> With only an ordinary CMOS flip flip this current isn't very
> high at low frequencies, but one can add a capacitor to the
> output of the flip-flop, increasing the current as desired.
> Even with the added capacitor, the circuit is still simple and
> arguably is becoming even more cute, but sadly both versions
> have a serious flaw. In the i = C V f formula, V is the flip-
> flop's supply voltage. This means the frequency-measuring
> voltage dropped across the R-C in series with the logic supply
> reduces the V in our formula, destroying the circuit's linearity.
> The larger the output voltage of the cute f > V converter, the
> worse this effect. :>(
> To solve this problem, one must create a supply voltage that
> doesn't change with load, and measure its current load, using
> a circuit something like this,
> . Linear Frequency to Voltage Converter
> . +12 etc. __
> . ---+---------------|+ \ A2
> . | | \___________ OUT
> . +----||----, | /
> . | R +----|-_/\ difference
> . '---/\/\---+ | amplifier,
> . | | INA105, etc.
> . __ | gnd
> . ,----|+ \ A1 D
> . | | >----G Q1 Vout = f R C Vcc/2
> . | ,-|-_/ S 2n7000
> . | | | Where C includes the
> . | '------------+--||-- gnd flip-flop's internal
> . | | low-Z FF capacitance.
> . | +5 Vcc | bypass
> . --+--------|>]----+ Trim R to calibrate.
> . __|__ C
> . ,--|D Q|--||-- gnd
> . | | |
> . f-in ---|--|> |
> . | |___/Q|--,
> . |___________|
> In this circuit A1 serves to maintain the flip flop at the
> same voltage as the rest of the logic, and a Schottky diode
> prevents possible latchup at power turn-on. The flip flop's
> power pins are directly bypassed with a capacitor to supply
> the potentially-high switching currents during transitions.
> The current for the flip flop with its output-enhancing load
> capacitor is provided by MOSFET Q1, and is sensed by resistor
> R, which can be trimmed to calibrate for the imprecise value
> of C and the loading effect of the internal resistors in A2.
> Although my circuit should work well, and fixes the serious
> problem in Elektor's simple circuit, its complexity prevents
> it from being called cute.
And since Burr-Brown and Analog Devices both have nice, fast and very
linear voltage to frequency and frequency to voltage converters, why
fill up half a board re-inventing the wheel?
Bill Sloman, Nijmegen