- Initial specifications
- Better stability/lower noise voltage reference
- Modification for current shunt resistors and current source
- Replace terminal block with triaxial ports
- Reversible terminal block
- Route active OHM guarding to guard
- Low-noise power regulation circuitry
- -10V and other hidden calibrations
- Isolated battery power supply
Let’s first take a look on HP/Agilent/Keysight 3458A meter specification with high-stability DC reference – Option 002 and measured data for temperature coefficient.
|DC Voltage||100mV||1 V||10 V||100V||1000 V|
|Accuracy (1y), Rdg ppm + Range ppm||5+3 (0.0005%)||4+0.3 (0.0004%)||4+0.05 (0.0004%)||6+0.3||6+0.1|
|Temperature coefficient, ppm||1.2+1||1.2+0.1||0.5+0.01||2+0.4||2+0.04|
|Measured tempco, ppm||1.2+1||1.2+0.1||0.5+0.01||2+0.4||2+0.04|
|RMS noise at NPLC100, Range ppm||0.2||0.02||0.01||0.02||0.01|
|AC Voltage||10mV||100mV||1V||10 V||100V||1000 V|
|Best accuracy (1y), Rdg ppm + Range ppm||200+110 (0.02%)||70+20 (0.007%)||200+20 (0.02%)||400+20 (0.04%)|
|Temperature coefficient, ppm||30+200||25+1|
|Measured tempco, ppm||30+200||25+1||25+1||25+1||25+1||25+1|
|Bandwidth frequency (max accuracy)||1 Hz to 10MHz (40Hz to 1kHz)|
|Resistance 4W||10 Ω||100 Ω||1 KΩ||10 KΩ||100 KΩ||1 MΩ||10 MΩ||100 MΩ||1 GΩ|
|Accuracy (1y), Rdg ppm + Range ppm||15+5||15 (0.0012%)||10+0.5 (0.0010%)||15+2||50+10||500+10 (0.05%)||5000+10 (0.5%)|
|Source current||10 mA||1 mA||100 µA||50 µA||5 µA||500 nA|
|Temperature coefficient, ppm||3+1||3+1||3+0.1||3+1||20+20||100+20||1000+20|
|Measured tempco, ppm||3+1||3+1||3+1||3+1||3+1||3+1||20+20||100+20||1000+20|
|RMS noise at NPLC100, Range ppm||0.3||0.3||0.03||0.045||0.06||3.6||36|
|DC Current||100 nA||1 µA||10 µA||100 µA||1 mA||10 mA||100 mA||1 A|
|Accuracy (1y), Rdg ppm + Range ppm||30 + 400||20+40||20+10||20+8||20+5||20+5||35+5||110+10|
|Temperature coefficient, ppm||10+200||2+20||10+4||10+3||10+2||10+2||25+2||25+3|
|Measured tempco, ppm||100+200||2+20||10+4||10+3||10+2||10+2||25+2||25+3|
|Shunt resistance (burden voltage)||545.2kΩ(55mV)||45.2kΩ(45mV)||5.2kΩ(55mV)||720 Ω(75mV)||100 Ω(100mV)||10 Ω(100mV)||1 Ω(250mV)||0.1 Ω (<1.5V)|
|RMS noise at NPLC100, Range ppm||2.5||0.25||0.025||0.025||0.025||0.025||0.025||0.025|
|AC Current||100 µA||1 mA||10 mA||100 mA||1 A|
|Accuracy (1y), Rdg ppm + Range ppm||600+300||600+200||600+200||600+200||800+200|
|Temperature coefficient, ppm||20|
Table 1: 3458A key specifications
This is helpful to determine narrow spots which can attempted for improvement.
Better stability/lower noise voltage reference
Noise of the reference voltage as an important source – one would need to improve there too, to have lower noise not only for a zero reading. Also many signal sources have at least comparable noise – so the only thing to do there would be to reduce the noise bandwidth by having 2 amplifiers + ADCs and do a kind of commutating auto zero – so not missing any of the signal. So this is an obvious possible factor of 2 in speed – but at a price.
Modification for current shunt resistors and current source
Start with easy part – DCI function. Why easy? Because it’s simple component swap.
I have broken A1 board, from which I removed original R213, R212, R211, R207, R209, R206 resistors and tested their TCRs to establish our baseline expectations.
- R212 1 ohm – +8.3 ppm/K Datalog
- R211 9 ohm – +4.7 ppm/K Datalog
- R207 40K “reference” VHP101 – +1.4 ppm/K Datalog
I don’t have much better replacement for 40K VHP101, so decided to leave that one alone for now, as we as 90 ohm (1mA range) and 4.53K shunts (10uA range), for but replaced rest with better resistors:
- R213 replaced with VPG foil shunt 319790 0.1 ohm 9W, measured TCR +0.55 ppm/K
- R212 replaced with VPG VPR221Z 1 ohm, measured TCR -0.86…-1.47ppm/K
- R211 replaced with AE foil resistor 9 ohm, measured TCR +1.5…+0.94 ppm/K
- R209 replaced with S102 foil resistor 634 ohm, measured TCR -1.2…1.25 ppm/K
- R206 replaced with CADDOCK USF340 resistor 500Kohm, measured TCR +0.78 ppm/K
After installing A1 board back, no SELF-TEST errors detected, ACAL ALL passed clean too.
Running GPIB app now collecting same 100mA running thru both meters from HP 3245A (LTZ-powered).
3458B : -4.65 ppm/K measured, 25ppm/K spec.
3458X : -1.88 ppm/K measured, 25ppm/K spec
3458B : -10.6 ppm/K measured, 25ppm/K spec.
3458X : +61.6 ppm/K measured, 25ppm/K spec
3458B : +3.36 ppm/K measured, 10ppm/K spec.
3458X : +10.92 ppm/K measured, 10ppm/K spec
Retest 10mADC range only confirms bad result
3458B : +3.47 ppm/K measured, 10ppm/K spec.
3458X : +12.35 ppm/K measured, 10ppm/K spec
3458B : +0.55 ppm/K measured, 10ppm/K spec.
3458X : +5.48 ppm/K measured, 10ppm/K spec
3458B : -1.42 ppm/K measured, 10ppm/K spec.
3458X : -0.45 ppm/K measured, 10ppm/K spec
No problem with 100µA range at all.
3458B : <100 ppm/K measured, 210ppm/K spec.
3458X : <100 ppm/K measured, 210ppm/K spec
Error/tempco of 100nA range mostly delivered from amplification stage, so this measurement have no meaning. It matches unmodified 3458, so replacement of 500K resistor was for nothing and not required.
DCV comparison, 100mV range
3458B : +1.72 ppm/K measured, 2.2ppm/K spec.
3458X : +0.63 ppm/K measured, 2.2ppm/K spec
DCV comparison, 1000mV range
3458B : -0.2 ppm/K measured, 1.3ppm/K spec.
3458X : -0.128 ppm/K measured, 1.3ppm/K spec
As we can see, DCV 100mV and 1V ranges are good and cause zero problems, and not cause of DCI excessive tempco. We can close A3 tempco effect theory for now.
Original A1’s resistors TCR tests:
- R211 9R ohm from A1 – -1.38 ppm/K Datalog – measured directly by K2002
- R209 634R ohm from A1 – +4.65 ppm/K Datalog – measured directly by K2002
- R206 500 Kohm from A1 – +1.86 ppm/K Datalog – measured directly by K2002
- R212 1 ohm shunt from A1 – +1.7 ppm/K Datalog – measured directly by K2002
- R213 0.1 ohm HP 03458-82501-1 shunt from A1 – -6.2…-13 ppm/K Datalog – measured directly by K2002
- R213 0.1 ohm HP 03458-82501-1 shunt from A1 – 7.5 ppm/K Datalog – measured by Keithley 182-M 1000mA test current, from 25°C to 35°C
|Resistor||Original A1||New resistor||Range||Original TCI||Test result X1|
|R213 0.1R||+7.50 ppm/K||+0.55 ppm/K||1 A||-4.65 ppm/K||-1.88 ppm/K|
|R212 1R||+1.70 ppm/K||-0.86…-1.47 ppm/K||100 mA||-10.6 ppm/K||+61.6 ppm/K|
|R211 9R||-1.38 ppm/K||+1.5…+0.94 ppm/K||10 mA||+3.36 ppm/K||+12.35 ppm/K|
|R210 90R||Unchanged||unchanged||1 mA||+0.55 ppm/K||+5.48 ppm/K|
|R209 634R||+4.65 ppm/K||+1.2…+1.25 ppm/K||100 uA||-1.42 ppm/K||-0.45 ppm/K|
|R208 4.53K||Unchanged||unchanged||10 uA||DNT||Not tested|
|R207 40K||Unchanged||unchanged||1 uA||DNT||Not tested|
|R206 500KR||+1.86 ppm/K||+0.78 ppm/K||100 nA||<100 ppm/K||<100 ppm/K|
Replace terminal block with triaxial ports
Reversible terminal block
Route active OHM guarding to guard
Low-noise power regulation circuitry
-10V and other hidden calibrations
Except documented Zero, 10V and 10 KΩ calibrations meter does accept additional less known calibration values:
|0.1 VDC||-0.9 … 0.9||0.1V calibration|
|-10 VDC||-12 … -1||-10V calibration|
|10 VDC||1 … 12||10V calibration (standard)|
|1000 Ω||1000 – 12000 ohm||10 KΩ (standard)|
Isolated battery power supply
Modified: July 22, 2017, 1:30 p.m.
- Tale 1 : Little jumper, or sharks in voltage references world
- xDevs.com : Study of temperature coefficient on 150 precision resistors
- xDevs.com HP 3458A restoration worklog (first unit)
- Repair log of reader's HP 3458A precision DMM
- 3458A Multimeter CLIP with schematics
- CERN: High Precision Current Measurement in Power Converters., Miguel Cerqueira Bastos, CAS 2014