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increasing the source impedance for ADC

PostPosted: Sat Jan 18, 2014 9:52 pm
by timbo
Hi,
I'm using one of the LPC2136's ADC to read the voltage on an RC Servo potentiometer, but its impedance appears to be too low. Do you have any suggestions how to buffer the impedance in this case. I don't have the schematic of the servo's circuit board to know exactly what I'm up against. I need to scrounge around for an op amp since it's one way to solve this issue. But I was hoping for other ideas.

Thanks

Re: increasing the source impedance for ADC

PostPosted: Sun Jan 19, 2014 2:08 pm
by Daniel
I assume you want to increase the impedance of the ADC or decrease the source impedance of the servo. An op-amp is the proper way of doing it but sometimes we can fake it since the ADC input impedance has two terms, a DC or low frequency term and a term that is there when the input is sampled. The datasheet for the LPC213x family recommends a external source impedance for the signal being measured of 40k Ohm or less (p32) and shows an internal resistance of 20k for the ADC pin. It also shows a few capacitors. If the pot in the servo has a value of greater than 40k then you need a proper buffer. If it is less than 40k then it is likely that the switching nature of the ADC is loading the pot via the switching capacitors of the ADC. In that case you can 'lower' the source impedance by placing a capacitor in parallel with the wiper of the pot (input pin to ground on the micro). Even 100n will swamp the load current peaks from the ADC.

However, a quick search on the web shows that the pot in the servos is almost universally a 5k pot and the ADC should not load that materially. It may affect the accuracy of the servo somewhat (if it is a really good servo) but the ADC will be accurate. More likely, you need to be careful with the ground wires. Servos draw a lot of current when loaded and if you were not careful in getting a really good ground to the servo, the voltage measured will change as the servo moves or is loaded due to voltage drops in the ground connection.

If you need a buffer a good choice for the op-amp is a Micrcohip MCP6023 since it works properly at 5V and 3.3V, is available in DIP packages and has rail to rail inputs and output and is accurate enough to not to reduce the accuracy of the ADC.

I hope this helps.
Daniel