of Microprocessors

DIGITAL-TO-ANALOG AND ANALOG-TO-DIGITAL CONVERTERS 243
GAIN TRIM
CURRENT
INPUT
FROM
CURRENT
OUTPUT
DAC
R2
OFFSET
TRIM
Fig. 7-15. Fast, inexpensive current-to-voltage converter
resulting fractional volt signal to standard levels with good speed, since less
frequency compensation is needed. Noise pickup could be a problem,
though, if this technique is used with high-resolution DACs.
The current-to-voltage converter configuration shown in Fig. 7-15 is
probably the best, really inexpensive circuit available for most applications of
current-output DACs. The "three-for-a-dollar" LM301 op-amp using feedforward
compensation easily gives under 3-f-tsec settling times, normally
obtainable only with more costly "high-speed" op-amps. Although the effective
zero impedance of the amplifier summing node eliminates output capacitance
slowdown, in practice C1 is usually needed to keep the DAC output
capacitance from making the op-amp unstable. Gain and offset errors are also
easily trimmed out in this circuit because the adjustments do not interact.
Some types of CMOS DACs are affected by output amplifier bias current,
which is approximately 100 nA for the LM301. FET-type op-amps, such as
the LF356, are then required and C1 is mandatory with a value in the 10-30
pF range. These also require that the amplifier's own offset be trimmed to
zero in order to minimize differential linearity errors.
Number Coding
Several different binary codes are in common use with DACs. All of the
example circuits given previously were unipolar, that is, gave output voltages
between zero and a Vref or output currents between zero and Iref. One
way to obtain a bipolar voltage output would be to use the dual emitterfollower
bipolar transistor switch shown in Fig.7-12 with a bipolar reference
supply. The result would be a voltage output DAC that would swing between
- Vref and + Vref- 1LSB (switches connected to R-2R ladder). For
example, all zeroes would give - Vref, 10000 ... would give zero volts, and
11111 . . . would give one step less than + Vref. Such a code is called offset
binary because it is equivalent to a plain unsigned binary (positive output
only) DAC output between zero and +2Vre!shifted down by Vrefas in Fig.