The circuit of

Figure 6 may be used if it is desired to obtain

precise reference voltages. The LMC6082 in this circuit was

chosen for its low offset voltage, low voltage rail-to-rail capa-

bility and low cost.

Since the current flowing through the sense lines (those lines

associated with V

there is negligible voltage drop across any resistance in se-

ries with these sense pins and the voltage at the inverting in-

put of the op-amp accurately represents the voltage at the

top (or bottom) of the ladder. The op-amp drives the force in-

put, forcing the voltage at the ends of the ladder to equal the

voltage at the op-amp’s non-inverting input, plus any offset

voltage. For this reason, op-amps with low V

LMC6081 and LMC6082, should be used for this application.

Voltages at the reference sense pins (V

should be within the range specified in the Operating Ratings

table (2.6V to 3.8V for V

V

be able to source sufficient current into the V

sink sufficient current from the V

is at its minimum value of 850

Ω.

The reference voltage at the top of the ladder (V

take on values as low as 1.0V above the voltage at the bot-

tom of the ladder (V

voltage at the bottom of the ladder (V

ues as low as 1.7V and as high as 2.8V. However, to mini-

mize noise effects and ensure accurate conversions, the to-

tal reference voltage range (V

minimum of 1.0V and a maximum of 2.2V.

DS101064-19

FIGURE 5. Improved Low Component Count Reference Biasing

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