DAC1006/1007/1008 — Simple Hookup for a “Quick Look”
(Continued)
4.2 Op Amp Bias Current & Input Leads
The op amp bias current (I
B
) CAN CAUSE DC ERRORS.
BI-FET
™
op amps have very low bias current, and therefore
the error introduced is negligible. BI-FET op amps are
strongly recommended for these DACs.
The distance from the I
OUT1
pin of the DAC to the inverting
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 ba-
sic 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-chromium
(Si-Cr) thin film R-2R ladder which is deposited on the sur-
face oxide of the monolithic chip. As a result, there is no
parasitic diode connected to the V
REF
pin as would exist if
diffused resistors were used. The reference voltage input
(V
REF
) can therefore range from −10V to +10V.
The digital input code to the DAC simply controls the position
of the SPDT current switches, SW0 to SW9. A logical 1 digi-
tal input causes the current switch to steer the available lad-
der current to the I
OUT1
output pin. These MOS switches op-
erate in the current mode with a small voltage drop across
them and can therefore switch currents of either polarity.
This is the basis for the 4-quadrant multiplying feature of this
DAC.
5.1.1 Providing a Unipolar Output Voltage with the
DAC in the Current Switching Mode
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
FB
, from the output of the op
amp to the inverting (−) input. Now, when current is 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 out-
put voltage of the op amp is forced to a voltage given by:
V
OUT
= −(I
OUT1
xR
FB
)
Notice that the sign of the output voltage depends on the di-
rection of current flow through the feedback resistor.
In current switching mode applications, both current output
pins (I
OUT1
and I
OUT2
) should be operated at 0 V
DC
. This is
accomplished as shown in
Figure 3. The capacitor, C
C
, is
used to compensate for the output capacitance of the DAC
and the input capacitance of the op amp. The required feed-
back resistor, R
FB
, is available on the chip (one end is inter-
nally tied to I
OUT1
) and must be used since an external resis-
tor will not provide the needed matching and temperature
tracking. This circuit can therefore be simplified as shown in
Figure 4, where the sign of the reference voltage has been
changed to provide a positive output voltage. Note that the
output current, I
OUT1
, now flows through the R
FB
pin.
DS005688-9
FIGURE 1. Basic Logic Threshold Loop
www.national.com
9
PrintDate=1998/11/17 PrintTime=11:38:09 46711 ds005688 Rev. No. 4
cmserv
Proof
9
