Reply-To: "Ralph Naylor"
From: "Ralph Naylor"
References: <email@example.com> <firstname.lastname@example.org> <email@example.com>
Subject: Re: frequency to voltage converter
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Sat, 18 Jan 2003 01:47:33 GMT
NNTP-Posting-Date: Fri, 17 Jan 2003 20:47:33 EST
"Bill Sloman" wrote in message
> Winfield Hill wrote in message
> > 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
Ageed. Also quick-and-dirty National LM2907/2917
(http://www.national.com/ds/LM/LM2907.pdf), throw the whole thing together
for a few bucks. It's been around for at least 25 years, as evidenced by
the breaker points in some of their suggested applications.
Ralph in NH