I think the standard circuit for a current reference will be more than good enough.  Put the bandgap voltage into the non-inverting input of an op amp.  Have the opamp drive the gate of a PMOS device.  Then take the current of the PMOS device into a resistor to ground.  Route that voltage (the drain of the PMOS) to the inverting input of the op amp.  The negative feedback will set the current through the PMOS device to Vref/R.  Choose R to give you the current you want and then connect other PMOS devices in parallel to generate as many currents as you need.

Carl

vp1953,

Correct me if I'm wrong, but are you referring to the CMOS bandgap reference Tsividis co-presented in the JSSC in June 1979?  If so, I think it is redundant since AnalogDE already has got a bandgap-derived voltage available.  AnalogDE would still need some loop to generate a constant current based on that voltage.

Come to think of it, the simplest solution might be to connect the available bandgap voltage directly to a PMOS device with a really small W/L ratio to lower its gm.  Since the AnalogDE's accuracy requirements are very loose, that may be good enough, even with PVT variations.

Another idea is to use a self-biased current source.  You can see one in Gray, Hurst, Lewis, and Meyer for example.  The downside of that one is that you would need a startup circuit.  I'm thinking the single PMOS device would be the easiest.  Give it a try and let me know if it works!

Carl

I like the idea of connecting one of my VREFs to a weak PMOS... I'll try that and see what I get.

Dan:  I'm worried about supply variation since I have a large VDD operating range.

Thank you all for your suggestions.

Rob,

I think we had the same idea about putting the 1.3V across a long device.  I guess whether it is PMOS or NMOS depends on the supply and whether AnalogDE needs a source or a sink.  I liked your idea about degenerating the device.  AnalogDE should probably do that regardless.

If the 1.3V Vgs that Rob's method provides is too much to control the current, you can also try using a resistive voltage divider to drop to down before you apply it to the gate.  Make sure that the resistors are pretty big so you don't load your bandgap.

AnalogDE: come to think of it, since you have a large VDD operating range maybe Rob's method with the degeneration is the ticket.

Hi vp1953,

vp1953 wrote on Dec 2^nd, 2010, 4:51pm:


My assumption was that AnalogDE would be getting the bandgap voltage externally somehow (like from a voltage reference input to the chip or even a reference IP) so it would not be possible to access an internal node and mirror an internal current.  If AnalogDE has access to the bandgap block, by all means mirroring that current is the easiest way to go.  Good idea.

AnalogDE, I would say if you have access to the internal nodes of the bandgap, do as vp1953 has suggested, otherwise apply the bandgap voltage to a large device with a very small W/L ratio and with source degeneration.

Good luck,
Carl

I think I'm going to go with a weak NMOS with gate connected to my VREF.  
What does source degeneration buy me and how do I go about sizing the resistor?  (I need to review my theory...)  

Also, for a high valued resistor it'll have to be implemented as an nwell resistor -- so another source of variation in process and tempco....

Thanks all.

Hi analogDE

you should use ur bgr to bias the NMOS transistor and connect  two or more PMOS transistor on it upto VDD  making drain and gate terminal short for PMOS .Then u can generate the desired current source by adjusting the W/L ratio of other transistor which gate is connected to the PMOS  gate.(U should used the gate of PMOS to bias the other transistor to get current reference follow the concept of current mirror )