mixed_signal wrote on Feb 1st, 2012, 6:53pm:Hi,
I am designing a smart temperature sensor with following specs:
1. On current 3uA (sensor+ADC)
2. Inaccuracy with no calibration 1C
3. Resolution 0.1c
4. Range -20C to 30C
5. Sampling rate 1 sample/minute. (too slow!!)
I assume that you have a bandgap reference provided. The bandgap value will likely be the largest source of the error so hopefully for you it is someone else's issue.
Quote:1. What is the typical value of the bias current of the BJTs? i have power constraint. So planning to use 100nA.
It depends on the BJT area so you'll want to use a pretty small device - ~2x2 um^2. As I hope you know, if you operate two bipolars at two different current densities (say the ratio is M) then the difference between the emitters is kT/q*ln(M). That has no process dependence so you'll be in great shape.
Except that is only true at a proper bias. To make sure you are in a good operating region, sweep the current and measure the voltage difference and note where it falls off at the high current and low current. Obviously you want to operate somewhere in the middle. Try this at hot/cold
Mismatch of your mirrors will have to be managed. BJTs match very well.
Quote:2. Two BJTs can be kept at different current densities by different current ratio or emitter area ratio? Why in temperature sensors usually current ratio is preferred and emitter area is kept constant.
Probably because that is how the guy before did it. I don't think there is any inherent advantage, but the bipolars can be relatively large so it can save area. You can also double sample the same BJT at two difference current densities to get the delta Vbe using only one BJT and a two value current source. I should have charged you for that one, but I won't tell you how to do it w/ a single current source
If you can do it, put "N" BJTs in series or in Darlington configuration so that your differential voltage is N*kT/q*ln(M) instead of using an opamp to gain up the kT/q*ln(M) signal. Guess I'm in a "give it away" mood today.
Quote:3. What type of opamp is best suitable for bandgaps/ temp sensors?
What is the typical gain, UGB etc. of the op amp? May I use opamp in subthreshold? Moreover, the op amp offset causes lot of inaccuracy. So, I need to use auto zero or chopper. It also should have high PSRR.
You'll want the lowest noise/highest gain possible, which is IMO a two stage Miller type opamp (except compensate it by putting the compensation between the gate/source of the output device for good AC PSRR).
Chop it. Barrel shift your mirror elements if they give you issues. Use as much current as you need and shut it down for 59 seconds. There, I just cut your power by a factor of 60 and solved your mismatch problems.
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If you give me a VPN account and some money I can probably do it for 300nA on average.