Current source "without" temp drift

Hi,

I'd like to make a High precision current source with excellent temp drift (10 to the power of -6 or at best better than that). Current: about 0.5 Ampere

At the moment I have an AD624 combined with a Vishay resistor in mind. How can I go further? Is there a place for me to look up stuff like this?

Thanks

Matthias

  • 0
    •  Analog Employees 
    on Jun 11, 2011 10:40 PM

    Hello Matthias,

    Do you mean that you need a 0.5A current source which has less than 1ppm/C drift? That is very hard to achieve. It is a highly complex problem. Even if you have one or two very good components, you have to take into account and minimize (or control) every error source in your circuit (and pc board!). Then there is also the possibility to control the temperature so there is no temperature drift within a reasonably narrow operating temperature range. Can you provide more details of your end application? Are there additional requirements such as noise, bandwidth, etc?

    Gustavo

  • Hello Gustavo,

    we would like to create a constant magnetic field, which we adjust once. It should be set by voltage or PC. Very low noise is pretty cool, but drift is more important. There are no other additional requirements, maybe not extremely expensive

    Matthias

  • 0
    •  Analog Employees 
    on Jun 13, 2011 5:58 AM

    Matthias,

    There are a few options and things to consider that might simplify your design. Some of those include a controlled environment and the ability of tracking the performance of your circuit.

    1. Is this a circuit for production or a single experiment?

    2. For how long does the circuit need to operate?

    3. Do you have a narrow operating temperature range? Is this going to be done in a controlled environment?

    4. Will you have the ability of tracking long-term stability of your circuit output with at least one more laboratory standard?

    5. Is it possible to relax your requirements to say, 5ppm/K?

    And a couple more questions about the source:

    6. When you say "current source" do you mean an actual source or a "current sink"?

    7. If you do mean a current source, what's the maximum voltage to which it needs to operate (a.k.a. compliance voltage)?

    8. I assume your load is quite constant. Is that correct?

    Gustavo

  • This is going to be very challenging. If you use a reasonably large value resistor, e.g. 0.2 ohms (200 milliohms) then you will drop 100 mV across it. This means your sense amplifier must have a drift of less than 0.1uV. Opamps like the AD8628, TLC2652, AD8571 or LTC1050 have a typical drift of less than 0.01uV/C, so they may be suitable over a temperature range of 10 C. The trouble is that the resistor will be dissipating 50mW, and even a very good quality foil resistor has a coefficient of 5 ppm at full power. This means you will need to run the resistor at perhaps 2% of its rating.

    If you reduce the sense resistance, the resistor may be more stable but the contribution from the sense amplifier becomes more significant.

    Electrical joints also become very troublesome as thermocouple effects can swamp 0.1uV.

  • @Gustavo

    1. it's for experiment

    2. min. 2h, max 2 days

    3. it's on the field measurement, it must be close to stuff, which heats up to about 60 degree Celsius (in the lab at 20 degree), when we switch it on

    4. no, this should be the laboratory standard

    5. 5ppm/K isn't fine, but if better is to hard to achieve, it's maybe ok

    6-8 the source should drive an air-core coil with 9 Ohm (max. 4.5 Volt), the field around isn't changing

    @Oliver

    see 5.

    Matthias