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DAC1007 Datasheet(PDF) 8 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
No. de pieza DAC1007
Descripción Electrónicos  uP Compatible, Double-Buffered D to A Converters
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Fabricante Electrónico  NSC [National Semiconductor (TI)]
Página de inicio  http://www.national.com
Logo NSC - National Semiconductor (TI)

DAC1007 Datasheet(HTML) 8 Page - National Semiconductor (TI)

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42 Op Amp Bias Current
Input Leads
The op amp bias current (IB) CAN CAUSE DC ERRORS BI-
FETTM 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 IOUT1 pin of the DAC to the inverting
input of the op amp should be kept as short as possible to
prevent inadvertent noise pickup
50 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
51 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 VREF pin as would exist if
diffused resistors were used The reference voltage input
(VREF) can therefore range from b10V to a10V
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-
able ladder current to the IOUT1 output pin These MOS
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
511 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 RFB from the output of the
op amp to the inverting (b) 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 output voltage of the op amp is forced to a voltage
given by
VOUT eb(IOUT1cRFB)
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 (IOUT1 and IOUT2) should be operated at 0 VDC This is
accomplished as shown in
Figure 3 The capacitor CC is
used to compensate for the output capacitance of the DAC
and the input capacitance of the op amp The required feed-
back resistor RFB is available on the chip (one end is inter-
nally tied to IOUT1) and must be used since an external
resistor will not provide the needed matching and tempera-
ture tracking This circuit can therefore be simplified as
DIGITAL INPUT CODE
FIGURE 2 Current Mode Switching
OP AMP CC pF Rj ts mS
LF356
22
%
3
LF351
24
%
4
LF357
10
24k
15
TLH5688 – 10
FIGURE 3 Converting IOUT to VOUT
8


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