Datasheet AD558 (Analog Devices) - 7

AD558 tDH VOUT DATA 2.0V 16 tDS INPUTS AD558 0.8V VOUT SENSE 15 RL 2.0V CS OR CE RP-D = 2x VEE 0.8V NEGATIVE VEE (in k
Ω
) t SUPPLY W
Figure 9. Improved Settling Time
1/2 LSB
available, bipolar output ranges may be achieved by suitable
DAC V OUTPUT
output offsetting and scaling. Figure 10 shows how a ± 1.28 volt output range may be achieved when a –5 volt power supply is
tSETTLING
available. The offset is provided by the AD589 precision 1.2 volt
tW = STORAGE PULSE WIDTH = 200ns MIN
reference which will operate from a +5 volt supply. The AD544
tDH = DATA HOLD TIME = 10ns MIN
output amplifier can provide the necessary ± 1.28 volt output
tDS = DATA SETUP TIME = 200ns MIN
swing from ± 5 volt supplies. Coding is complementary offset
tSETTLING = DAC OUTPUT SETTLING TIME TO
±
1/2 LSB
binary. Figure 7. AD558 Timing
5k
Ω
V = 0V TO +2.56V OUT USE OF V +5V OUT SENSE 0.01
µ
F
Separate access to the feedback resistor of the output amplifier
16
allows additional application versatility. Figure 8a shows how
5k
Ω
AD558 15
I × R drops in long lines to remote loads may be cancelled by
AD544 14
putting the drops “inside the loop.” Figure 8b shows how the
4.53k
Ω
V 0.01
µ
F O 12 13 +1.28 TO
separate sense may be used to provide a higher output current
1.5k
Ω
500
Ω
–1.27
by feeding back around a simple current booster.
BIPOLAR V –5V IN OFFSET AD589 ADJUST VOUT 16 AD558 INPUT CODE V V –1.2V OUT V OUT SENSE OUT 15 0V TO +10V 00000000 +128V 0.01
µ
F 4.7k
Ω
10000000 0V 12 13 14 R 11111111 –1.27V L GND GAIN –5V SELECT
Figure 10. Bipolar Operation of AD558 from ±5 V Supplies a. Compensation for I × R Drops in Output Lines
MEASURING OFFSET ERROR VCC
One of the most commonly specified endpoint errors associated with real-world nonideal DACs is offset error.
VOUT 16 2N2222 AD558
In most DAC testing, the offset error is measured by applying
VOUT SENSE 15 VOUT
the zero-scale code and measuring the output deviation from 0
0V TO +2.56V 12 13 14
volts. There are some DACs, like the AD558 where offset errors
GAIN RL
may be present but not observable at the zero scale, because of
GND SELECT
other circuit limitations (such as zero coinciding with single- supply ground) so that a nonzero output at zero code cannot be read as the offset error. Factors like this make testing the b. Output Current Booster AD558 a little more complicated. Figure 8. Use of VOUT Sense By adding a pulldown resistor from the output to a negative
OPTIMIZING SETTLING TIME
supply as shown in Figure 11, we can now read offset errors In order to provide single-supply operation and zero-based at zero code that may not have been observable due to circuit output voltage ranges, the AD558 output stage has a passive limitations. The value of the resistor should be such that, at zero “pull-down” to ground. As a result, settling time for negative voltage out, current through the resistor is 0.5 mA max. going output steps may be longer than for positive-going output steps. The relative difference depends on load resistance and
OUTPUT
capacitance. If a negative power supply is available, the
AMP
negative-going settling time may be improved by adding a pull-
0.5mA 16 VOUT
down resistor from the output to the negative supply as shown
–V
in Figure 9. The value of the resistor should be such that, at
15 VOUT SENSE
zero voltage out, current through that resistor is 0.5 mA max.
14 VOUT SELECT BIPOLAR OUTPUT RANGES 13 AGND
The AD558 was designed for operation from a single power supply and is thus capable of providing only unipolar (0 V to +2.56 V and 0 V to 10 V) output ranges. If a negative supply is a. 0 V to 2.56 V Output Range –6– REV. B