LTC1605
10
1605fd
For more information www.linear.com/LTC1605
APPLICATIONS INFORMATION
Internal Voltage Reference
The LTC1605 has an on-chip, temperature compensated,
curvature corrected, bandgap reference, which is factory
trimmed to 2.50V. The full-scale range of the ADC is
equal to (±4 • V
REF
) or nominally ±10V. The output of the
reference is connected to the input of a unity-gain buffer
through a 4k resistor (see Figure 3). The input to the buffer
or the output of the reference is available at REF (Pin 3).
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 CAP (Pin 4). The
CAP 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 REF pin and the
CAP pin should each be decoupled with a capacitor to
filter wideband noise from the reference and the buffer
(2.2µF tantalum).
Offset and Gain Adjustments
The LTC1605 offset and full-scale errors have been trimmed
at the factory with the external resistors shown in Figure4.
This allows for external adjustment of offset and full scale in
applications where absolute accuracy is important. See Figure
5 for the offset and gain trim circuit. First adjust the offset
to zero by adjusting resistor R3. Apply an input voltage of
–152.6mV (–0.5LSB) and adjust R3 so the code is changing
between 1111 1111 1111 1111 and 0000 0000 0000 0000.
The gain error is trimmed by adjusting resistor R4. An input
voltage of 9.999542V (+FS – 1.5LSB) is applied to V
IN
and R4
is adjusted until the output code is changing between 0111
1111 1111 1110 and 0111 1111 1111 1111. Figure 6 shows
the bipolar transfer characteristic of the LTC1605.
–
+
1605 • F03
INTERNAL
CAPACITOR
DAC
BANDGAP
REFERENCE
V
ANA
4k
2.2μF
CAP
(2.5V)
2.2μF
REF
(2.5V)
4
3
Figure 3. Internal or External Reference Source
Figure 4. ±10V Input Without Trim
+
5
4
3
2
1
2.2μF
+
2.2μF
33.2k
1%
10V INPUT
200Ω
1%
V
IN
AGND1
REF
CAP
AGND2
LTC1605
1605 • F04
Figure 5. ±10V Input with Offset and Gain Trim
+
5
4
3
2
1
2.2μF
+
2.2μF
33.2k
1%
10V INPUT
200Ω
1%
V
IN
AGND1
REF
CAP
AGND2
LTC1605
1605 • F05
576k
R4
50k
R3
50k
5V
INPUT VOLTAGE (V)
0V
OUTPUT CODE
–1
LSB
1605 • F06
011...111
011...110
000...001
000...000
100...000
100...001
111...110
1
LSB
BIPOLAR
ZERO
111...111
FS/2 – 1LSB–FS/2
FS = 20V
1LSB = FS/65536
Figure 6. LTC1605 Bipolar Transfer Characteristics
DC Performance
One way of measuring the transition noise associated
with a high resolution ADC is to use a technique where
a DC signal is applied to the input of the ADC and the
resulting output codes are collected over a large number
of conversions. For example in Figure 7 the distribution of
