PROBLEM 099 · Beginner · The resistor-string DAC
Eight Steps
A resistor string for eight voltage levels
The problem
Build a resistor string: eight equal resistors in series between the net “vrefp” (the test bench puts 1.8 V there) and ground, with the junctions named “t1” (the one next to ground) up to “t7” (the one next to “vrefp”). Choose the resistor value: the string should draw about 90 µA from the reference.
The seven taps divide the reference into eight equal steps of 225 mV: the voltage of tap k is k/8 of the reference. The string is the voltage divider generalised: accuracy depends only on resistor matching. A flash ADC compares the input with each tap; a string DAC connects one tap to the output with a switch.
The test bench
- Supply: not needed.
- The test bench puts 1.8 V on “vrefp” and reports the worst difference between the tap voltages and the ideal k/8 · 1.8 V, in LSB (225 mV), and the current drawn from the reference.
Numbers to hit
- Worst tap error at most 0.02 LSB
- Current from the reference between 70 µA and 110 µA
Design it yourself
Draw the circuit on a schematic canvas, size every transistor, and see it simulated in your browser with ngspice and the SkyWater SKY130 models. Every transistor lights up green, gold or red by operating region as you move the sliders.
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