pico power op amp suggestion pls

Why do you need a follower for that?

Cheers

Phil Hobbs

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IIRC the input impeadance of the ADC input is about 10M. What is the purpose of that voltage follower? How are you going to measure the lithium cell, which is normally ~3.6V, by the AVR ADC powered by 3V?

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

er

Is this AVR run fron the Lithium? Will the op-amp be?

If the answer to either is "no", then you have a problem with what happens when the power to the connected parts is off.

If the answer is yes, I think you have a different problem. You need to bring the battery voltage down to something within the span of an ADC powered from the battery.

You may just need a MOSFET to connect the battery to the measurement circuit. The amount of time spent measuring it, can be so low that the small current drain won't matter.

An output pin of the micro can drive the MOSFET.

If the AVR is running from the battery, you can measure it's own supply voltage without any external A2D. We do it all the time, upto

1% accuracy with precision resistors.

Everything is powered from the lithium- AVRmega48P, nrf905 and the AVR has on-board ADC and reference.The data sheet says o/p impedance for ADC should be

What you want is 1meg, 2 meg divider with bypass capacitance, say 0.1,

0.047 uf ceramic. This provides (transiently) the low impedance required by your ADC without burning excess power and without an opamp.

PN2222A

V+ -+- | | .-. | | 10k | | '-' .------- | | o------->--| Ain | | .-. | | | 22k | | | | AVR uC '-' | | | | | '--|| | .->|| | +--|'---

Yup. Don't sample too often (that'll load it too much) and that's a fine idea.

er

You don't say which specific AVR you are using but on many of the devices you can measure the supply voltage with no external components or additional battery drain.

For the ADC input channel select the reference to be AVCC and the input to be 1.3V (internally connected from the internal band-gap reference - ADC MUX setting 0xE on the ATMEGA8).

When you do a conversion you are then measuring the supply voltage with VCC as the reference, the reverse of the usual arrangement. By appropriate calculation you can determine the supply voltage.

kevin

If the micro is powered directly from the battery, you can use a voltage divider and either a port pin (float or drive high to make the power dissipation go away) or a transistor to pull down the low end of the voltage divider.

Eg.

.-------------+--------------------+ | | .-. | ' | | | .---------. | | 15K 1% | | Vdd | '-' | | | | | | ADC in|---------------+ Vx ' | | | | + | | .-. --- | | | | - | Port pin|--------+ | | 30.1K 1% | | Vss | | '-' | '---------' | | | | +------+ | | | | -------------+

Vx = 0.667 * Vbatt when port pin is low, Vx = 1.00 * Vbatt when port pin is high or floating

Source impedance (port pin low) ~= 15K || 30.1K = 10.0K

If it takes 50usec to make a measurement and you measure 10 x per second, the average current draw of the voltage divider would be the same as a 90M ohm resistor across Vdd-Vss (about 40nA), which ought to be negligible. Of course you'll also want to shut down the ADC and reference when they are not being used.

For better accuracy, there are other techniques that could be used, but I think this will likely suffice for your purposes (but as always, check).

Best regards, Spehro Pefhany

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You could simply use a voltage detection chip like a TPS3809L30DBVR

Output goes high when the batt voltage falls below 2.64V, perfect for a 3V lithium batt low voltage detect.

Dave.

Correction, output goes LOW when the batt voltage falls below 2.64V.

Dave.