Need to design precision 5mA constant current source (wide t

[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]

On 22 Nov 2004 14:41:04 -0800, stephendeese@hotmail.com (stevo) wrote:

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

---
Does one end of the bridge need to be connected to ground?

--
John Fields

 

[stevo]

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


John Fields <jfields@austininstruments.com> wrote in message news:<opt4q09847ovdcgaiid6ej5i36d52etbbo@4ax.com>...

On 22 Nov 2004 14:41:04 -0800, stephendeese@hotmail.com (stevo) wrote:

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

---
Does one end of the bridge need to be connected to ground?

 

[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 <jpopelish@rica.net> wrote in message news:<41A293D6.463EBE3E@rica.net>...

stevo wrote:

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?

 

[John Popelish]

stevo wrote:

John Popelish wrote:
stevo wrote:
(snip)
Ohter info : V avalible +15V; bridge resistance ~ 850 ohms

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

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.

--
John Popelish

 

[John Popelish]

stevo wrote:

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?

--
John Popelish