LTC1857 LINEAR [Linear Integrated Systems], LTC1857 Datasheet - Page 12

• Manufacturer Part Number: LTC1857
• Description: 8-Channel, 12-/14-/16-Bit, 100ksps SoftSpan A/D Converters with Shutdown
• Manufacturer: LINEAR [Linear Integrated Systems]

APPLICATIO S I FOR ATIO
LTC1857/LTC1858/LTC1859
buffer or the output of the reference is available at V
(Pin 15). The internal reference can be overdriven with an
external reference if more accuracy is needed. The buffer
output drives the internal DAC and is available at REFCOMP
(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.
For minimum code transition noise the V
REFCOMP pin should each be decoupled with a capacitor
to filter wideband noise from the reference and the buffer.
UNIPOLAR / BIPOLAR OPERATION
Figure 4a shows the ideal input/output characteristics for
the LTC1859. The code transitions occur midway between
12
4.096V
Figure 4a. Unipolar Transfer Characteristics (UNI = 1)
2.5V
0.1µF
Figure 3. Internal or External Reference Source
111...111
111...110
111...101
111...100
000...011
000...010
000...001
000...000
1µF
15
16 REFCOMP
10µF
0V
V
REF
UNIPOLAR
ZERO
1LSB = FS
LSB
1
1.6384X BUFFER
U
8k
65536
INPUT VOLTAGE (V)
REFERENCE
U
2.5V
W
FS – 1LSB
CAPACITIVE DAC
12-/14-/16-BIT
1859 F4a
REF
pin and the
U
1859 F03
REF
successive integer LSB values (i.e., 0.5LSB, 1.5LSB,
2.5LSB, … FS – 1.5LSB). The output code is natural binary
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
the offset applied to the “–” input. For single-ended inputs,
this offset should be applied to the COM pin. For differen-
tial inputs, the “–” input is dictated by the MUX address.
For unipolar zero offset 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 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.
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
Figure 4b. Bipolar Transfer Characteristics (UNI = 0)
011...111
011...110
000...001
000...000
111...111
111...110
100...001
100...000
–FS/2
1LSB = FS
65536
INPUT VOLTAGE (V)
BIPOLAR
LSB
–1
ZERO
0V
LSB
1
FS/2 – 1LSB
1859 F4b
185789f