LTC1857/LTC1858/LTC1859
W U U
U
APPLICATIO S I FOR ATIO
buffer or the output of the reference is available at VREF
(Pin 15). The internal reference can be overdriven with an
external reference if more accuracy is needed. The buffer
outputdrivestheinternalDACandisavailableatREFCOMP
(Pin 16). The REFCOMP pin can be used to drive a steady
DC load of less than 2mA. Driving an AC load is not
recommended because it can cause the performance of
the converter to degrade.
successive integer LSB values (i.e., 0.5LSB, 1.5LSB,
2.5LSB,…FS–1.5LSB).Theoutputcodeisnaturalbinary
with 1LSB = FS/65536. Figure 4b shows the input/output
transfer characteristics for the bipolar mode in two’s
complement format.
FULL SCALE AND OFFSET
In applications where absolute accuracy is important,
offset and full-scale errors can be adjusted to zero during
a calibration sequence. Offset error must be adjusted
before full-scale error. Zero offset is achieved by adjusting
theoffsetappliedtothe“–”input. Forsingle-endedinputs,
this offset should be applied to the COM pin. For differen-
tial inputs, the “–” input is dictated by the MUX address.
Forunipolarzerooffseterror,apply0.5LSB(actualvoltage
will vary with input span selected) to the “+” input and
adjust the offset at the “–” input until the output code
flickers between 0000 0000 0000 0000 and 0000 0000
0000 0001 for the LTC1859, between 00 0000 0000 0000
and 00 0000 0000 0001 for the LTC1858 and between
0000 0000 0000 and 0000 0000 0001 for the LTC1857.
8k
15
1µF
V
REF
2.5V
2.5V
REFERENCE
12-/14-/16-BIT
CAPACITIVE DAC
1.6384X BUFFER
16 REFCOMP
4.096V
0.1µF
1859 F03
10µF
Figure 3. Internal or External Reference Source
For minimum code transition noise the VREF pin and the
REFCOMPpinshouldeachbedecoupledwithacapacitor
tofilterwidebandnoisefromthereferenceandthebuffer.
For bipolar zero error, apply –0.5LSB (actual voltage will
vary with input span selected) to the “+” input and adjust
the offset at the “–” input until the output code flickers
between 0000 0000 0000 0000 and 1111 1111 1111
1111 for the LTC1859, between 00 0000 0000 0000 and
UNIPOLAR / BIPOLAR OPERATION
Figure 4a shows the ideal input/output characteristics for
theLTC1859. Thecodetransitionsoccurmidwaybetween
FS
FS
1LSB =
111...111
111...110
111...101
111...100
011...111
011...110
1LSB =
65536
65536
BIPOLAR
ZERO
000...001
000...000
111...111
111...110
UNIPOLAR
ZERO
000...011
000...010
000...001
000...000
100...001
100...000
0V
1
LSB
–1 0V
1
FS – 1LSB
–FS/2
FS/2 – 1LSB
LSB
LSB
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
1859 F4a
1859 F4b
Figure 4a. Unipolar Transfer Characteristics (UNI = 1)
Figure 4b. Bipolar Transfer Characteristics (UNI = 0)
185789f
12
Linear Systems [ Linear Systems ]
