1. Home
  2. Electronics Design
  3. Precision current source

Precision current source

Hi all, I've using a voltage reference->Resistor-> opamp feedback to make a current source. Like figure 10 here.

formatting link
The reference is the REF102, but I'm using an opa134 for the opamp. (Currents down to 10 nA so I wanted a fet opamp.)

Now someone wants to use it where noise might be more of an issue, so I figured the first thing was to measure the noise.

Well, the dang thing is singing at ~1.8 MHz. Not that loud... about 100 uV p-p, but still this seems like bad form. Any thoughts on how I might squash this?

TIA George H. (Then maybe I'll ask about how to filter the noise further.. I was thinking about a cap. multiplier hanging on the output)

Reply to
George Herold
Loading thread data ...

Current ranges from 10 nA to 1 mA. There is no noticeable change in the oscillations as the current is changed. (Which kinda bothers me.)

George H.

Reply to
George Herold

Nasty twist on Howland. Might try capacitor from Vout to Trim. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

The obvious issue with the REF102 circuit is that there's no capacitor in parallel with R3 - typically one uses a couple of pF - to compensate for the input capacitance to ground at the inverting input (typically another couple of pF).

The need for it varies with the bandwidth of the op amp (and the input capacitance) but leave it out can give you the kind of oscillation you mention.

The major source of noise in a voltage reference is often the reference voltage generator, which is why the REF102 circuit has 7k resistor which you can use with a capacitor to low-pass filter the noise coming out of the voltage reference.

6.2V zener-based references are essentially avalanche diodes and can be quite noisy at low currents.

AOE3 is a full bottle on the subject, and has a couple of very useful pages of advice.

--
Bill Sloman, Sydney
Reply to
Bill Sloman

This is stable:

formatting link

or use a bootstrap source

formatting link

formatting link

You could lowpass filter the bootstrap voltage, to reduce noise, but that decreases the current source impedance at higher frequencies. Ultimately the Johnson noise will getcha.

Reply to
John Larkin

That was a good idea, but did nothing.

George H.

Reply to
George Herold

Thanks Bill, I think this was Jim T's suggestion for some C from out to the trim.

George H.

Reply to
George Herold

Yeah.. I don't really want to do a different circuit. I'm wondering if my feedback opamp (opa134) is just too fast. I'll see if I can dig up a slower FET.

George H.

Reply to
George Herold

Resistor in series between op amp output and -in Series RC between -in and +in

Adjust to give the op amp a unity gain cross of ~500 kHz.

This works by increasing the noise gain.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Hmm OK thanks.. how to reduce the BW of a unity gain buffer... I didn't know that.

Say R = 10 k C ~100pf.. and play around there.

I found a 1 MHz opamp (vs 8MHz for opa134 before) and that reduced the signal by a factor of two or so.

To be honest, this might be my measurement error. I was trying to filter the oscillations with an RC low pass before the load and couldn't... Scratch scratch... George H.

Reply to
George Herold

The bad news about that trick is that it reduces the high-frequency input impedance.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Hi Phil, Well if I did it right... then it didn't work at all for me. (I saw gain peaking at intermediate frequencies.) (Maybe I'll have time to spice it tonight.)

But the whole discussion is a red herring. The sorta long leads going to an amp were giving me some unknown capacitance... or maybe a ground inductance problem.. (I'm not sure.) And causing the amp to wig out at ~2MHz. I cleaned it up and now can measure the noise... Well after I put the circuit back together. Grumble most of a day gone to operator error.

George H.

Reply to
George Herold

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.