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low-cost high-voltage electrometer amplifier

Discussion in 'Electronic Design' started by Winfield Hill, Oct 10, 2007.

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  1. Here's a nice example of a circuit that pulls itself
    up by its bootstraps - a high-voltage opamp follower
    featuring electrometer-style high input impedances.

    My design is meant to replace expensive high-voltage
    opamps, like the PA97 offered by Apex, with low-cost
    superior-performance low-voltage opamps, like those
    offered by Analog Devices (Stephan Goldstein should
    enjoy this one). My goal is for an amplifier with
    an offset voltage under 65uV and less than 1pA input
    current, yet operating over a 430-volt signal range.

    IN ___| \___ OUT - within
    <1pA |__/ 65uV of input

    For example, Analog Devices' ad8605 is a quiet (8.5nV)
    fast (10MHz, 5V/us) sot-23 opamp that meets two out
    of three of my specs, and yet only costs $1, qty 100.,2877,AD8605,00.html

    The ad8605 fails the 400V spec, of course. Here's the
    basic idea for my solution, which features two elegant
    450V Supertex depletion-mode MOSFETs. This is just
    the basic idea; my full circuit takes 28 to 35 parts.

    input | |--+ dn3145
    protection _____|________|
    \__ _|_ | |
    IN --|__|---|+ \ | _|_,
    | >--' /_\ 5.1V
    ,--|-__/ |
    |____|______________|____ OUT
    ad805 |

    Well, whaddya think?
  2. John Larkin

    John Larkin Guest

    Cute. I've seen opamps whose rails tracked the opamp Vout, intended to
    give huge cmrr's.

    Post the full circuit!

  3. I am curious about your 'input protection',
    would there not be a very short input current spike if you
    applied a step from 0 to +400V ?
    That could cause problenms in some circuit.
  4. Win,
    I'm interested. Considering the cost of an
    APEX part, there is some slack in building
    the function from scratch with 35 parts.

  5. John Larkin

    John Larkin Guest

    OK, what am I missing?

  6. Yes indeed. For example, with its infinite Zin and
    vanishingly-low input current, if a probe is used to
    measure +400 volts, after removal the amplifier will
    simply stay there. Then if the probe is touched to
    ground, the amplifier's input will experience an
    instantaneous -400-volt spike.

    The easy fix is a pair of back-to-back diodes at
    the input to the circuit's "supply-ground" output,
    along with some current limiting input resistance.
    I chose two 1k resistors in series at the input,
    along with one more between the diode node and the
    opamp to limit its current during the fault. The
    low value was chosen to preserve the opamp's 8.5nV
    voltage noise. But 2k input resistance allows for
    a 200mA fault current, so I added a depletion-mode
    MOSFET circuit to further limit the fault current
    to about +/- 0.5 to 1mA.

    I used my favorite Supertex LND150 or LND250 parts.
    The protection diodes are low-leakage low-capacitance
    BJT base-collector junctions.

    500V protection
    1k | | 1k
    o--/\/\--, ,---+--/\/\--+---, ,--/\/\--+--/\/\-- opamp
    _|__|_ | | _|__|_ |
    ---, | | ,--- +--|>|--,
    |___| _______| | '--|<|--+--- out

    This protection circuit can withstand a continuous
    input connection to +400 volts, while the circuit's
    high-voltage power source is off.

    I also added parts to the rest of the circuit to
    protect the MOSFETs, etc., during the fast slewing.
  7. Well, in its favor, the APEX does have lots of
    features mine lacks, such as high output drive.
    But they would make for expensive HV-voltmeter
    high-impedance voltage followers. We needed
    two inputs for a differential measurement, so
    I decided to design this circuit. There are
    several applications where it can be used.
  8. John Larkin

    John Larkin Guest

    You could also do a flying-capacitor thing, with a dpdt relay, if
    static measurements were enough. It could report the differential
    input almost 1:1, or you could dump into a bigger cap and get a
    voltage-divider effect.

  9. Guest

    You could allways get a service manual for a Tek 7a13 plugin and look
    how they did it.
  10. John Larkin

    John Larkin Guest

    Or a type Z.

  11. Winfield

    Winfield Guest

    What, tubes? That's going backwards, not forwards!
  12. Winfield

    Winfield Guest

    My input current-limiting circuit, transient
    protection for the MOSFETs, a small isolated
    capacitor to absorb fault charge, a protected
    guard circuit, and series resistors to limit
    the fault currents if a MOSFET fails.

    You have an extra zener with cap, unless you
    want to add an active PMOS pulldown circuit
    to increase negative slew-rate capability.
    That's my next version and pushes the parts
    count up to 38.

  13. John,
    and the switches for the high voltage being ?
    There are several conditions to it. Such as
    low leakage, low charge injection. Win was
    talking about an electrometer input, meaning
    there cannot 1uF at the input.

  14. The depletion Fets are single sourced ? I only
    heard about one manufacturer once, and even
    forgot about it. I'd prefer enhancement MosFets.
    A schematic is one thing and some estmations,
    simulations or measurements are another.
    What are expectable slew rates ?

  15. Beautiful, this one I will keep on the harddisk :)
    Thank you.
  16. Winfield

    Winfield Guest

    There are at least three manufacturers, although
    Supertex is the most prolific and committed one.
    Unh Huh. Be my guest trying to create a simple
    effective circuit like this with an enhancement-
    mode MOSFET. You need ugly things, like a pullup
    resistor that has to provide sufficient current
    over a two-decade range of signal voltages, or a
    HV current source (which is best made using a
    depletion-mode transistor), etc. I've done it,
    multiple times, but it's a true pleasure to be
    able to use depletion-mode instead. Supertex is
    a reliable manufacturer, and now that Mouser is
    stocking their parts there's little excuse to
    avoid them. We all work with sole-source parts
    all the time. Just maintain a good inventory.
    What! You question the operational efficacy of my
    circuits? :) Actually, this one has been reduced
    to elegant practice, with a nice SMD-component PCB.
    Pullup slew rates are very fast, but in my simple
    class-A circuit (again below), the fixed pulldown
    current source is a limitation, against the combined
    Coss of the pullup + pulldown MOSFETs, plus any load
    capacitance, such as a triaxial guard cable, etc.

    input | |--+ dn3145
    protection _____|________|
    \__ _|_ | |
    IN --|__|---|+ \ | _|_,
    | >--' /_\ 5.1V
    ,--|-__/ |
    |____|______________|____ OUT
    ad805 |

    If the load is just a resistive divider, the circuit
    can be reasonably fast. Supertex dn3145 and dn2540
    are candidate MOSFETs, with a Coss of about 12pF at
    25V, so let's calculate for C = 25pF (for the pullup
    plus pulldown CS mosfets). dV/dt = i/C = 60V/us,
    which means it takes 6.6us to slew 400 volts, and
    can follow a 50kHz 400Vp-p sine wave.

    My full circuit, 2nd rev, adds a p-channel pulldown
    MOSFET, which I expect to double the negative slew rate
    and maintain this speed into a triax guard capacitance.
  17. John Larkin

    John Larkin Guest

    Sometimes pv optoisolators can be handy in situations like this.
    Nowadays, the idea of having 2 and 3 sources for everything just isn't

  18. John Larkin

    John Larkin Guest

    You can do some cool stuff with depletion fets, like very nice current
    limiters. We've never had a problem getting the Supertex parts.
    Certainly. I'd do all that if I had a reason to, and a target spec to
    meet. At present, I don't have either.
    You can do that math as well as I can.

    Do you always rain on ideas?

  19. John Larkin

    John Larkin Guest

    Some people can riff on ideas, and some people stamp them down as soon
    as they appear.

  20. Actually the preferrence was more towards something
    buyable, not against depletion as itself.

    Supertex ist having a strange policy. Their minimum
    order quantity apparently is a wheel of 1k pieces.
    Yes, they do samples and can be asked for lower
    quantities, but with our distributor in between
    they are somewhat unwieldy. Perhaps I should look
    at Mouser, which haven't ordered yet from.

    No offense intended
    That sounds fantastic

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