dwoodwar@undeed.uucp (Duncan R Woodward) (10/25/90)
hi Netters, Does anybody out there know of a neat circuit for a voltmeter with an input impedance of approximately 10 Giga ohms ? I am particularly interested in the input stages. The meter must be able to measure input voltages in the range 0 to 50v DC. Thanks in advance for any information. Duncan Woodward (Department of Electrical Engineering) University of Natal Durban South Africa email: dwoodwar@Daisy.EE.UND.AC.ZA internet: dwoodwar.UNDEED@f4.n494.Z5.fidonet.Org.
whit@milton.u.washington.edu (John Whitmore) (10/31/90)
In article <1990Oct25.080455.3910@undeed.uucp> dwoodwar@undeed.uucp (Duncan R Woodward) writes: >hi Netters, > >Does anybody out there know of a neat circuit for a voltmeter with an >input impedance of approximately 10 Giga ohms ? I am particularly >interested in the input stages. The meter must be able to measure >input voltages in the range 0 to 50v DC. Many MOS input op amps can reach 10 Gohm, but few if any can take 50V input range with respect to their power supply rails. So, there's an old trick; one bootstraps the regulated power supply from the op amp output to make a wide-range follower. +70V | | +-----+ | | R1 | | / +---| | \ (emitter of NPN transistor) CR1 | | _____ input +---| \_________ output --------|---|+ / | | ----- | +------|------+ | | CR2 | (emitter of PNP transistor) | / +----| | \ R2 | | | +------+ | -20V Choose zener diodes CR1 and CR2 according to the power supply rating of the op amp; probably two 5V zeners are best. The transistors will need to take up to 80V. For the op amp, I'd think of CA3140 (Harris) or LF441 or LM356 (National Semiconductor). The benefit of this circuit is that the common mode voltage is constant (so the bias current fluctuations of the op amp are not related to the input voltage). Choose any op amp with sufficiently low bias current, preferably with low power supply current requirements. John Whitmore whit@milton.u.washington.edu