dac1008 National Semiconductor Corporation, dac1008 Datasheet - Page 8

• Manufacturer Part Number: dac1008
• Description: Up Compatible, Double-buffered D To A Converters
• Manufacturer: National Semiconductor Corporation

4 2 Op Amp Bias Current
The op amp bias current (I
FET
the error introduced is negligible BI-FET op amps are
strongly recommended for these DACs
The distance from the I
input of the op amp should be kept as short as possible to
prevent inadvertent noise pickup
5 0 ANALOG APPLICATIONS
The analog section of these DACs uses an R-2R ladder
which can be operated both in the current switching mode
and in the voltage switching mode
The major product changes (compared with the DAC1020)
have been made in the digital functioning of the DAC The
analog functioning is reviewed here for completeness For
additional analog applications such as multipliers attenua-
tors digitally controlled amplifiers and low frequency sine
wave oscillators refer to the DAC1020 data sheet Some
basic circuit ideas are presented in this section in addition to
complete applications circuits
5 1 Operation in Current Switching Mode
The analog circuitry Figure 2 consists of a silicon-chromi-
um (Si-Cr) thin film R-2R ladder which is deposited on the
surface oxide of the monolithic chip As a result there is no
parasitic diode connected to the V
diffused resistors were used The reference voltage input
(V
The digital input code to the DAC simply controls the posi-
tion of the SPDT current switches SW0 to SW9 A logical 1
digital input causes the current switch to steer the avail-
REF
TM
) can therefore range from
op amps have very low bias current and therefore
OUT1
B
) CAN CAUSE DC ERRORS BI-
Input Leads
pin of the DAC to the inverting
b
REF
10V to
pin as would exist if
FIGURE 3 Converting I
a
FIGURE 2 Current Mode Switching
10V
DIGITAL INPUT CODE
8
OUT
able ladder current to the I
switches operate in the current mode with a small voltage
drop across them and can therefore switch currents of ei-
ther polarity This is the basis for the 4-quadrant multiplying
feature of this DAC
5 1 1 Providing a Unipolar Output Voltage with the
A voltage output is provided by making use of an external
op amp as a current-to-voltage converter The idea is to use
the internal feedback resistor R
op amp to the inverting (
entered at this inverting input the feedback action of the op
amp keeps that input at ground potential This causes the
applied input current to be diverted to the feedback resistor
The output voltage of the op amp is forced to a voltage
given by
Notice that the sign of the output voltage depends on the
direction of current flow through the feedback resistor
In current switching mode applications both current output
pins (I
accomplished as shown in Figure 3 The capacitor C
used to compensate for the output capacitance of the DAC
and the input capacitance of the op amp The required feed-
back resistor R
nally tied to I
resistor will not provide the needed matching and tempera-
ture tracking This circuit can therefore be simplified as
to V
DAC in the Current Switching Mode
OUT1
OUT
and I
OP AMP C
LF356
LF351
LF357
OUT1
FB
OUT2
V
OUT
is available on the chip (one end is inter-
) and must be used since an external
) should be operated at 0 V
e b
C
22
24
10
pF R
b
(I
OUT1
) input Now when current is
OUT1 c
2 4k
%
%
FB
j
output pin These MOS
ts mS
from the output of the
1 5
R
3
4
FB
)
TL H 5688 – 10
DC
This is
C
is