# Not enough current for voltage divider?

I'm trying to use a HIH4030 humidity sensor, where the output value is provided to a XBee.

Considering that output sensor max output voltage is around 4V and the XBee ADC only supports 1,2V, I took some time to choose the correct resistors for a voltage divider to get a decent ratio. I end up choosing a 120 ohm and a 270 ohm resistors to get a ratio of 4V to 1V.

And in this case I wasn't getting any values on my XBee. When I measured the signal, I had about 31mV going out the voltage devider when I should have around 800mV.

Now, correct me if I'm wrong (and there's a good chance that's the case), but the reason for these low values are due to the low total resistance value (R1+R2 = 390ohm) and the low current value going out the sensor. The HIH4030 specs say that the current supply varies from 200uA to 500uA, so I'm assuming that from this sensor I won't get higher currents than 500uA. Is this a correct assumption?

Anyway, taking this assumption as the correct one, through ohms law,

V=RI. V=390*0.0005, thus V=195mV which is hardly enought.

I was considering getting higher resistance values, at least to a total of R=1,2/0.0005.

Am I proceeding correctly?

## 1 Answer

Yes, you're on the right track. The values in your voltage divider should be driven mainly by the characteristics of the ADC on the XBee.

Typically, the ADC on a microcontroller requires a source impedance no higher than about 10 kΩ. You could use 27 kΩ and 12 kΩ in your divider; the resulting source impedance would be about 8.3 kΩ, well within spec.

The load on the sensor would be 39 kΩ, drawing just over 100 µA at 4V.

• It's the parallel combination of the two divider resistors: $$\frac{1}{1/12k + 1/27k} = 8.3k$$ – Dave Tweed May 1 '14 at 1:05
• This probably is dumb question, but... why the parallel combination? Aren't the resistors on voltage divider in series? – cvicente May 1 '14 at 8:21
• They are in series from the point of view of the sensor, which is why it sees a 39k load. However, as long as we stay within its current limits, we can treat the sensor as a voltage source, which means that its output impedance is low. This means that the effective source impedance "seen" by the ADC input is the two resistors in parallel. – Dave Tweed May 1 '14 at 11:17
• Don't confuse the input impedance of the XBee (the load that it presents to the source) with the souce impedance that it can tolerate while still meeting its perforance specifications; these are two different numbers. Note that the input impedance shifts from 10M to 1M when taking a sample, a delta of 9M; this implies that your source impedance needs to be less than 9K if you want this shift to induce less than a 1% error. – Dave Tweed May 1 '14 at 16:26
• That sounds more like a drive specification, applicable when those pins are used as outputs. Probably irrelevant to your usage. – Dave Tweed May 2 '14 at 15:44