The DAC buddy

Frequent design idea contributor, Peter Demchenko, recently published “Extending the resolution of a peripheral DAC”. In it, Peter describes an inverting dual-supply amplifier circuit that he says is intended “…to improve the resolution of a peripheral DAC. The circuit also reduces output resistance of the DAC and can reduce its offset.”

The circuit presented here (see the figure) takes a somewhat different approach to a similar goal. It employs a single supply, non-inverting topology optimized for parameters (e.g., output offset and reference voltages) that are more accurately typical of actual microcontroller DAC specs (e.g., from the tinyAVR® 1-series Table 36-31: DAC offset error = -40 mV to +20 mV).

RRIO noninverting op-amp needs only one supply-voltage polarity to tailor DAC output.

R1, R2, R5, and R8 provide DAC offset adjustment:

R1/R2 = Vdd/2/Voffset(max = 40 mV)
R8/R5 = 2R1/R2

R3 and R4 provide DAC span adjustment: R4/R3 = 1 for a 2:1 span adjustment range. R6 extends DAC resolution* to 9 bits via general purpose bit DIOa: R6 = 29R5Vdd/Vdac(max = 4.3 V). R7 extends DAC resolution* to 10 bits via DIOb: R7 = 210R5Vdd/Vdac(max).

Although the signal levels used here to calculate resistor values (e.g., 4.3 V DAC output span and 40 mV DAC offset) are specific to the tinyAVR® 1-series, similar parameters will apply to other combinations of microcontroller and on-chip analog peripherals, making the circuit easy to translate to different contexts.

*Note that sadly, resolution and monotonicity are not necessarily the same thing. DNL rules!

Stephen Woodward’s relationship with EDN’s DI column goes back quite a long way. Over 100 submissions have been accepted since his first contribution back in 1974.

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