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RobG

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Posts: 570
Bozeman, MT

Re: Crude current reference circuit
Reply #15 - Dec 07^th, 2010, 5:41am

Isabelle wrote on Dec 7^th, 2010, 3:07am:

Hello,
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.....

Hi Isabelle,
Yes, that has been suggested by Carl and myself (but I suggested an NMOS). I'm curious how you or Carl would connect the PMOS. With an NMOS it is simple: Gate to Vref, and source to GND, and take the current from the drain. But how would you connect a PMOS?

rg

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Frank Wiedmann Community Fellow Offline Posts: 678 Munich, Germany	Re: Crude current reference circuit Reply #16 - Dec 7^th, 2010, 6:32am Is it just me who finds it strange that Isabelle's contribution is an exact quote from carlgrace (reply #3) and that there is a link to a "Dogs for sale" website in her signature? Ken?
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RobG Community Fellow Offline Posts: 570 Bozeman, MT	Re: Crude current reference circuit Reply #17 - Dec 7^th, 2010, 6:35am Frank Wiedmann wrote on Dec 7^th, 2010, 6:32am: Is it just me who finds it strange that Isabelle's contribution is an exact quote from carlgrace (reply #3) and that there is a link to a "Dogs for sale" website in her signature? Ken? LOL... you are correct
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carlgrace

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Posts: 231
Berkeley, CA

Re: Crude current reference circuit
Reply #18 - Dec 7^th, 2010, 4:23pm

Isabelle wrote on Dec 7^th, 2010, 3:07am:

Hello,
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.....

I am so honored that my suggestion could be used by Isabelle in her spam.

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carlgrace

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Berkeley, CA

Re: Crude current reference circuit
Reply #19 - Dec 7^th, 2010, 4:27pm

RobG wrote on Dec 7^th, 2010, 5:41am:

Isabelle wrote on Dec 7^th, 2010, 3:07am:

Hello,
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.....

Hi Isabelle,
Yes, that has been suggested by Carl and myself (but I suggested an NMOS). I'm curious how you or Carl would connect the PMOS. With an NMOS it is simple: Gate to Vref, and source to GND, and take the current from the drain. But how would you connect a PMOS?

rg

Rob,

It totally depends on the VDD range and whether AnalogDE needs to source or sink a current. For example, if the BG voltage is 1.1 V, for instance, it might be challenging to keep an NMOS under control with such a large VGS. If VDD where 1.8 V, for instance you could connect it like you suggested AnalogDE connect the NMOS: Vref to gate, source to VDD, drain to load. Either way works, depending on the situation.

AnalogDE said the plan was to go with the weak NMOS. I think it will work.

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RobG

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Posts: 570
Bozeman, MT

Re: Crude current reference circuit
Reply #20 - Dec 8^th, 2010, 7:26am

carlgrace wrote on Dec 7^th, 2010, 4:27pm:

RobG wrote on Dec 7^th, 2010, 5:41am:

Isabelle wrote on Dec 7^th, 2010, 3:07am:

Hello,
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.....

Hi Isabelle,
Yes, that has been suggested by Carl and myself (but I suggested an NMOS). I'm curious how you or Carl would connect the PMOS. With an NMOS it is simple: Gate to Vref, and source to GND, and take the current from the drain. But how would you connect a PMOS?

rg

Rob,

It totally depends on the VDD range and whether AnalogDE needs to source or sink a current. �For example, if the BG voltage is 1.1 V, for instance, it might be challenging to keep an NMOS under control with such a large VGS. �If VDD where 1.8 V, for instance you could connect it like you suggested AnalogDE connect the NMOS: Vref to gate, source to VDD, drain to load. �Either way works, depending on the situation.

AnalogDE said the plan was to go with the weak NMOS. �I think it will work.

Oh, ok. I don't like doing stuff like that (even though it was strongly endorsed by a lady selling dogs

). The reason is that Vref is bypassed to ground so any AC power supply noise appears across the PMOS gate/source and gets put onto the current (I_ac=g_m*V_noise). Most applications can handle the variation with Vdd, but not the AC noise, especially when mismatches are considered.

The larger Vgs is actually an advantage since it lowers the g_m. It also steers the temp co towards PTAT, which is better for most circuits. There is also a point where the temp co is zero. This point has been around Vgs=1.2 V when I've played with it, although I'm sure it depends on the Vt of the device.

Adding resistive degeneration decreases the g_m even more.

That's my story, and I'm sticking to it �:P .

Rob
-----------
No I don't sell dogs.

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mowiehowie

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Posts: 26

Re: Crude current reference circuit
Reply #21 - Dec 8^th, 2010, 10:18am

Isabelle wrote on Dec 7^th, 2010, 3:07am:

Hello,
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.....

do you sell only raw dogs ? any hot dogs ?

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carlgrace

Senior Member

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Posts: 231
Berkeley, CA

Re: Crude current reference circuit
Reply #22 - Dec 8^th, 2010, 1:13pm

RobG wrote on Dec 8^th, 2010, 7:26am:

carlgrace wrote on Dec 7^th, 2010, 4:27pm:

RobG wrote on Dec 7^th, 2010, 5:41am:

Isabelle wrote on Dec 7^th, 2010, 3:07am:

Hello,
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.....

Hi Isabelle,
Yes, that has been suggested by Carl and myself (but I suggested an NMOS). I'm curious how you or Carl would connect the PMOS. With an NMOS it is simple: Gate to Vref, and source to GND, and take the current from the drain. But how would you connect a PMOS?

rg

Rob,

It totally depends on the VDD range and whether AnalogDE needs to source or sink a current. �For example, if the BG voltage is 1.1 V, for instance, it might be challenging to keep an NMOS under control with such a large VGS. �If VDD where 1.8 V, for instance you could connect it like you suggested AnalogDE connect the NMOS: Vref to gate, source to VDD, drain to load. �Either way works, depending on the situation.

AnalogDE said the plan was to go with the weak NMOS. �I think it will work.

Oh, ok. I don't like doing stuff like that (even though it was strongly endorsed by a lady selling dogs

). The reason is that Vref is bypassed to ground so any AC power supply noise appears across the PMOS gate/source and gets put onto the current (I_ac=g_m*V_noise). Most applications can handle the variation with Vdd, but not the AC noise, especially when mismatches are considered.

The larger Vgs is actually an advantage since it lowers the g_m. It also steers the temp co towards PTAT, which is better for most circuits. There is also a point where the temp co is zero. This point has been around Vgs=1.2 V when I've played with it, although I'm sure it depends on the Vt of the device.

Adding resistive degeneration decreases the g_m even more.

That's my story, and I'm sticking to it �:P .

Rob
-----------
No I don't sell dogs.

You're right Rob. �Your idea is the way to go. �I didn't think about the noise... I figured if AnalogDE had such a loose tolerance on the matching then most likely there wasn't a big requirement on the noise. �I do get your point. �Good luck getting the tempco hitting the match zero spot. �Reminds me of g_m stages in RF circuits that try to hit the magical zero third-order distortion point.

Carl

PS I'm pretty sure Isabelle would have sold you a dog pre-cooked if you paid her enough...

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