Precision Current Source

[behzad63]

Hi
How a precision current source is made inside a high end DMM .I mean how they use the same reference for voltage measurements and also use it for OHM measurement.
Thanks

[David Hess]

There is an "ohms converter" circuit which generates a precision current determined by the reference voltage and the input divider which does double duty so different ohms ranges use different currets.  With this current applied to the positive terminal, the voltmeter reads out ohms directly.

[zlymex]

Here is the schematics of 34401A Ohm current source.
This is a two step procedure to formalize and for multi-ranges, although one step is more stable for a single range current source.
In both steps, the circuit is quite typical consist of Vref+resistor+opamp+jfet.
The current source of first step generate either 175uA or 17.5uA depending on selection of R201 or R202, this will create a secondary voltage reference of 5V or 0.5V on that 28.57k resistor inside chip U102-D. The second step generate 1mA(and others) in a similar way by this 5V reference+5k resistor+U201B+Q202.

The circuit for 3458A works in a similar way, the first step generate 3V(actually is the difference of 13V and 10V) .

[behzad63]

Dear zlymex
Thanks for your time
Can you explain how dose the circuit work with detail?
Thanks a lot

[zlymex]

The first part of the current source(consisting of +7REF, U201-A, Q201 and U101-D) is a very typical one, and often called current sink, detailed explanation can be found here: http://www.learningaboutelectronics.com/Articles/Current-source-circuit.php
The only difference is that it uses depletion-type MOSFET instead of JFET, but functionally are the same.

The second part is very similar to the first, except the polarity is reversed, which can then be called true current source(DMM needs it), with the following differences:
-5V Vref (28.57k) is used(+V referred) instead of +7V reference(ground referred)
P-channel JFET is used instead of N-channel.
Add C210 for stability
Add CR210-A and R290 for lessen the output of U201-B.
 
The function of CR203-A is to drop the supply from +18V to +12V.
The square-pules-shaped-lines within U101 blocks are switches controlled by other logic circuits that may be open or close depending on the selection of current source value etc.

There exist true current source circuits that make use of ground referred Vref directly as well, such as Howland current pump, and is also single resistor range changeable. But IMO, this type of current source is not good for DMM use, because it is difficult to make it very precise especially at low current level. Refer to AN-1515 for detail: http://www.ti.com/lit/an/snoa474a/snoa474a.pdf 

[zlymex]

I had a look at the Ohm current circuit of 1281, it is quite simple.
The opamp will keep the voltage drop of range resistor R219p the same as the float voltage V1, so that the current is only an Ib away from the calculated value of V1/R219p.
This kind of current source/sink can be found at page 10 of LTC1050 ds http://cds.linear.com/docs/en/datasheet/1050fb.pdf

The float voltage V1 is done by so called flying cap reference, consisting of 4 switches and two(or three) capacitors of 470nF.

[ali_asadzadeh]

Dear zlymex
Thanks for shedding the light on us, is there a current mirror in U101.42 and U101-D? if so is there a generic part number for the devices? or should we build it with discrete components?

[zlymex]

Dear zlymex
Thanks for shedding the light on us, is there a current mirror in U101.42 and U101-D? if so is there a generic part number for the devices? or should we build it with discrete components?

Yes, there is a current mirror that IMO is used for self-test purpose only, thus it will be switched off during normal operation. You can safely ignore that part without scarifies any performance.

Current mirror of that sort is never precise, it cannot be used as accurate 1:1 current radio for measurement.

[ali_asadzadeh]

Dear zlymex
Hi,
what's the relationship between the resistor values and output current? I mean how they calculate the output current ranges, For example suppose that I want to set the output current to 5mA? (for the 34401A circuit)

[zlymex]

Dear zlymex
Hi,
what's the relationship between the resistor values and output current? I mean how they calculate the output current ranges, For example suppose that I want to set the output current to 5mA? (for the 34401A circuit)

Divide the secondary voltage of 5V with the output current required, you will get 5V/5mA=1k