15
LTC1599
sn1599 1599fs
A 16-Bit General Purpose Analog Output Circuit
Industrial applications often use analog signals of 0V to
5V, 0V to 10V, ±5V or ±10V. The topology in Figure 6 uses
an LTC1599 to produce a universal analog output, capable
of operation over all these ranges, with only software
configuration. High precision analog switches are used to
provide uncompromising stability in all ranges and matched
resistors internal to the LTC1599 are used, as well as a
configuration that minimizes the effects of channel resis-
tance in the switches. Note that in all cases the analog
switches have minimal current flowing through them. The
use of unbuffered analog switches in series with the
feedback/divider resistors would result in an error be-
cause of temperature coefficient mismatch between the
internal DAC resistors and the switch channel resistances,
as well as the channel resistance variation over the signal
range. Quad analog switch U3 (DG212B) allows configu-
ration of feedback terms and selection of the reference
voltage. Switch C allows the buffered reference voltage to
be injected into the summing node via Pin 5 (R
OFS
) for
bipolar outputs. When active, switch D places R
OFS
in
parallel with R
FB
, producing an output at full scale voltage
equal to the voltage at the REF pin of the LTC1599.
The other switches in U3 (A and B) are used to select the
10V reference produced by the LT1019, or 5V produced by
the R3 and R4 divider.
An inexpensive precision divider can be implemented
using an 8-element resistor array, paralleling four resis-
tors for R3 and four resistors for R4. Symmetry in the
interconnection of these resistors will ensure compensa-
tion for temperature gradient across the resistor array. An
APPLICATIONS INFORMATION
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alternative to a resistor divider is the LTC1043 switched
capacitor building block. It can be configured as a high
precision divide-by-2. Please consult the LTC1043 data
sheet for more information.
The NOR gate (U4) ensures that switches C and D are not
enabled simultaneously. This eliminates contention be-
tween the reference buffer and the output amplifier.
This topology can be modified to accept a high current
buffer following the LT1112, if higher output current levels
are required or difficult loads need be driven. Adjustment
of C
FB
’s value may be required for the buffer amplifier
chosen.
Note that the analog switches must handle the full output
swing in this configuration, but there is a variety of suitable
switches on the market including the LTC201. The DG212B
as shown is a newer generation part with lower leakage,
providing a performance advantage.
The DG333A, a quad single-pole, double-throw switch,
could be used for a 2-channel version similar to this
circuit. Alternatively, a single channel can be created with
the additional switches used as switched capacitor divide-
by-2, as shown on the LTC1043 data sheet. In choosing
analog switches, keep in mind the logic levels and the
signal levels required.
Table 1. Configuration Settings for the Various Output Ranges
V
OUT
MODE REFSEL BIPOLAR/UNIPOLAR GAIN
0V to 5V 1 0 0
0V to 10V 1 0 1
–5V to 5V 1 1 1
–10V to 10V 0 1 1
