A voltage divider and LMC6482 based DC amplifier for use with an ADC (v1)
Circuit Diagram and LMC6482 Pinout
DC Voltage divider
The design uses a decade voltage divider resistor network feeding an op amp configured for a DC gain of 10. I used a resistor chain of 2.7M, 270k, 27k and 3k, total of 3M, because I had several of each resistor to match a divider chain to aprox 0.3%. I used a digital meter with a 4000 max reading allowing each resistor and the next down to be measured on the same range. The meter is very linear, even though the basic accuracy is only quoted as 1%, one can match resistors on the one range to better than that.
Other resistors in a 1:9:90:900 ratio can be used, e.g. 2k, 18k, 180k and 1.8M which gives an input of 2M.
A 2nF capacitor was used across the input to reduce AC response. The input of 3Mohms, and the parallel capacitor of 2nf, gives a lowpass frequence response of aprox. 15Hz, which might be a little low for many purposes. A value of 200pF would give a freq. of 150Hz and a value 20pf a freq. of 1.5KHz.
LMC6482 based DC amplifier
The LMC6482 is a rail-to-rail input and output, dual OP AMP, so this circuit uses a single supply of 3.3v to 15v. The output does not go right down to ground, but to within 20mV according to spec - 10mV measured in my samples. This means that this AMP is not accurate for measuring very small DC voltages.
The resistors chosen for the x10 amp were also chosen from those I had available and again checked for a 9:1 match. Other resistors in a 9:1 ratio can be used.
Notes
Using a dual op amp allows 2 circuits to be built, each identical, on a piece of strip board, using header pins to allow selection of gain/range with a jumper, and for connections to and from the board, i.e. Supply V, Vin1, Vout1, Vin2 and Vout2.
The circuit might need a bypass capacitor of aprox. 100nf across the supply pins to the op amp. My circuit worked fine without.
I use the circuit to feed an ADC that has a range of 0v to 2048mV, so this circuit gives full scale ranges of 200mV, 2V, 20V and 200V. Although, for safety reasons, I would not connect anything over 50V. If you plan on using higher voltages take care in construction and make sure the components at the input are specified at the required voltages.
Tue Mar 26 22:55:54 GMT 2013 Jim Jackson <jj@franjam.org.uk>
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