Need to design precision 5mA constant current source (wide temp range).. need help

[stevo]

Hi,

I need to design a 5mA constant current source for a Wheatstone bridge application. The problem is the source needs to be +- 3% accurate and
hold that accuracy over a -40 C to +85 C temperature range.

I have thought about using a simple FET CCS, but don't have much
experience and am not sure if I can make it accurate enough.

Can any of you more experienced guys point me in the right direction?

Thank You
Stephen

Ohter info : V avalible +15V; bridge resistance ~ 850 ohms

 

[John Fields]

Hi,

I need to design a 5mA constant current source for a Wheatstone bridge
application. The problem is the source needs to be +- 3% accurate and
hold that accuracy over a -40 C to +85 C temperature range.

I have thought about using a simple FET CCS, but don't have much
experience and am not sure if I can make it accurate enough.

Can any of you more experienced guys point me in the right direction?

Thank You
Stephen

Ohter info : V avalible +15V; bridge resistance ~ 850 ohms

 

[John Popelish]

Hi,

I need to design a 5mA constant current source for a Wheatstone bridge
application. The problem is the source needs to be +- 3% accurate and
hold that accuracy over a -40 C to +85 C temperature range.

I have thought about using a simple FET CCS, but don't have much
experience and am not sure if I can make it accurate enough.

Can any of you more experienced guys point me in the right direction?

Thank You
Stephen

Ohter info : V avalible +15V; bridge resistance ~ 850 ohms

Are you sure that a constant voltage across the bridge would not
improve its accuracy?

 

[stevo]

I currently have the end of the bridge connected to ground, but I
suppose it wouldn't have to be.

 

[stevo]

The spec sheet for the bridge has the following:

Sensitivity tempco:
Vbridge= 8V (constant voltage) 0.3 %/C
IBridge = 5mA 0.06 %/C

So, it appears that the bridge is more stable w/ the constant current.

 

[John Popelish]

The spec sheet for the bridge has the following:

Sensitivity tempco:
Vbridge= 8V (constant voltage) 0.3 %/C
IBridge = 5mA 0.06 %/C

So, it appears that the bridge is more stable w/ the constant current.

But those values are very different excitation levels. 8 volts across
850 ohms gives an excitation current of 9.4 ma, and a bridge
dissipation of 75 mw, while 5 ma of bridge current gives a bridge
dissipation of 21 mw. A 4.25 volt excitation would be more equivalent
to the 5 ma excitation, and may improve on its stability.