DAC0806 NSC [National Semiconductor], DAC0806 Datasheet - Page 8

• Manufacturer Part Number: DAC0806
• Description: 8-Bit D/A Converters
• Manufacturer: NSC [National Semiconductor]

Figure 8 A high input impedance is the main advantage of
Application Hints
A negative reference voltage may be used if R14 is ground-
ed and the reference voltage is applied to R15 as shown in
this method Compensation involves a capacitor to V
pin 16 using the values of the previous paragraph The neg-
ative reference voltage must be at least 4V above the V
supply Bipolar input signals may be handled by connecting
R14 to a positive reference voltage equal to the peak posi-
tive input level at pin 15
When a DC reference voltage is used capacitive bypass to
ground is recommended The 5V logic supply is not recom-
mended as a reference voltage If a well regulated 5V sup-
ply which drives logic is to be used as the reference R14
should be decoupled by connecting it to 5V through another
resistor and bypassing the junction of the 2 resistors with
0 1 F to ground For reference voltages greater than 5V a
clamp diode is recommended between pin 14 and ground
If pin 14 is driven by a high impedance such as a transistor
current source none of the above compensation methods
apply and the amplifier must be heavily compensated de-
creasing the overall bandwidth
OUTPUT VOLTAGE RANGE
The voltage on pin 4 is restricted to a range of
0 4V when V
ods employed in the DAC0808
The negative output voltage compliance of the DAC0808 is
extended to
negative than
and load resistor of 2 5 k
yield a voltage output of 256 levels between 0 and
b
speed or power dissipation However the value of the load
resistor determines the switching time due to increased volt-
age swing Values of R
affect performance but a 2 5 k
settling time to 1 2 s (when all bits are switched ON) Refer
to the subsequent text section on Settling Time for more
details on output loading
OUTPUT CURRENT RANGE
The output current maximum rating of 4 2 mA may be used
only for negative supply voltages more negative than
due to the increased voltage drop across the resistors in the
reference current amplifier
ACCURACY
Absolute accuracy is the measure of each output current
level with respect to its intended value and is dependent
upon relative accuracy and full-scale current drift Relative
accuracy is the measure of each output current level as a
fraction of the full-scale current The relative accuracy of the
DAC0808 is essentially constant with temperature due to
4 980V Floating pin 1 does not affect the converter
b
EE
b
5V where the negative supply voltage is more
e b
10V Using a full-scale current of 1 992 mA
5V due to the current switching meth-
L
up to 500
between pin 4 and ground will
(Continued)
load increases worst-case
do not significantly
b
0 55 to
EE
b
8V
on
EE
8
the excellent temperature tracking of the monolithic resistor
ladder The reference current may drift with temperature
causing a change in the absolute accuracy of output cur-
rent However the DAC0808 has a very low full-scale cur-
rent drift with temperature
The DAC0808 series is guaranteed accurate to within
LSB at a full-scale output current of 1 992 mA This corre-
sponds to a reference amplifier output current drive to the
ladder network of 2 mA with the loss of 1 LSB (8 A) which
is the ladder remainder shunted to ground The input current
to pin 14 has a guaranteed value of between 1 9 and 2 1
mA allowing some mismatch in the NPN current source
pair The accuracy test circuit is shown in Figure 4 The 12-
bit converter is calibrated for a full-scale output current of
1 992 mA This is an optional step since the DAC0808 accu-
racy is essentially the same between 1 5 and 2 5 mA Then
the DAC0808 circuits’ full-scale current is trimmed to the
same value with R14 so that a zero value appears at the
error amplifier output The counter is activated and the error
band may be displayed on an oscilloscope detected by
comparators or stored in a peak detector
Two 8-bit D-to-A converters may not be used to construct a
16-bit accuracy D-to-A converter 16-bit accuracy implies a
total error of
which is much more accurate than the
tion provided by the DAC0808
MULTIPLYING ACCURACY
The DAC0808 may be used in the multiplying mode with
8-bit accuracy when the reference current is varied over a
range of 256 1 If the reference current in the multiplying
mode ranges from 16 A to 4 mA the additional error con-
tributions are less than 1 6 A This is well within 8-bit accu-
racy when referred to full-scale
A monotonic converter is one which supplies an increase in
current for each increment in the binary word Typically the
DAC0808 is monotonic for all values of reference current
above 0 5 mA The recommended range for operation with
a DC reference current is 0 5 to 4 mA
SETTLING TIME
The worst-case switching condition occurs when all bits are
switched ON which corresponds to a low-to-high transition
for all bits This time is typically 150 ns for settling to within
6-bit accuracy The turn OFF is typically under 100 ns
These times apply when R
Extra care must be taken in board layout since this is usually
the dominant factor in satisfactory test results when mea-
suring settling time Short leads 100
for low frequencies and minimum scope lead length are all
mandatory
g
LSB for 8-bit accuracy and 100 ns to
g
of one part in 65 536 or
L s
500
and C
g
F supply bypassing
0 019% specifica-
O s
LSB for 7 and
g
0 00076%
25 pF
g