[volt-nuts] Fluke 845 Modification LTC2057

starbook at uplink.net starbook at uplink.net
Sat Dec 28 20:33:49 EST 2013

On 12/28/2013 6:54 PM, Randy Evans wrote:
> I am in the process of designing an upgrade for a Fluke 845 Null meter to eliminate the opto-chopper, which doesn't work in most units today.  I am planning on using the Linear Tech LTC2054 zero drift op amp and power it with two 3V lithium batteries (which charge off the AC when plugged in) that should allow it to run for many hours on batteries only.  The only fly-in-the-ointment for my plan is to design a low power isolation amplifier for the recorder output that will work with the two 3VDC  lithium batteries. I have found isolation amplifiers that will work but they have issues such as requiring +/- 15VDC, are uni-polar only, or cost over $100 each.  I am still working the problem but I was hoping that someone on the forum might have a good idea for an isolation amplifier that works on +/- 3VDC, uses very low power, and will take in -1 to +1 VDC and output an isolated  -1 to +1 VDC.
> Randy Evans
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Hi Randy,

  Check the new Linear Tech LTC2057 zero drift op amp.
Power supply 4.75V to36V.

  * *Supply Voltage Range*
    *4.75V to 36V (LTC2057)*
    *4.75V to 60V (LTC2057HV)*
  * *Offset Voltage: 4?V (Maximum)*
  * *Offset Voltage Drift: 0.015?V/°C (Maximum, --40°C to 125°C)*
  * *Input Noise Voltage*
    *200nV_P-P , DC to 10Hz (Typ)*
    *11nV/?Hz, 1kHz (Typ)*
  * Input Common Mode Range: V^-- -- 0.1V to V^+ -- 1.5V
  * Rail-to-Rail Output
  * Unity Gain Stable
  * Gain Bandwidth Product: 1.5MHz (Typ)
  * Slew Rate: 0.45V/?s (Typ)
  * A_VOL : 150dB (Typ)
  * PSRR: 160dB (Typ)
  * CMRR: 150dB (Typ)
  * Shutdown Mode

John Staron

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