LTC1409_15 Datasheet, PDF (13/20 Pages) Linear Technology – 12-Bit, 800ksps Sampling A/D Converter with Shutdown

LTC1409
APPLICATI S I FOR ATIO
80
70
60
50
40
30
20
10
0
1
10
100
1000 10000
INPUT FREQUENCY (Hz)
LTC1409 • TPC09
Figure 10a. CMRR vs Input Frequency
ANALOG INPUT
±2.5V RANGE
1
+A_IN
2
– A_IN
0V TO 5V
RANGE
2.5V
1µF
LTC1409
3
V_REF
4
REFCOMP
10µF
5
AGND
LTC1409 • F10b
Figure 10b. Selectable 0V to 5V or ±2.5V Input Range
however, the bipolar zero error (BZE) will vary. The change
in BZE is typically less than 0.1% of the common mode
voltage. Dynamic performance is also affected by the
common mode voltage. THD will degrade as the inputs
approach either power supply rail, from 86dB with a
common mode of 0V to 75dB with a common mode of
2.5V or – 2.5V.
Differential inputs allow greater flexibility for accepting
different input ranges. Figure 10b shows a circuit that
converts a 0V to 5V analog input signal with no additional
translation circuitry.
Full-Scale and Offset Adjustment
Figure 11a shows the ideal input/output characteristics for
the LTC1409. The code transitions occur midway between
successive integer LSB values (i.e., –FS + 0.5LSB, –FS +
1.5LSB, –FS + 2.5LSB,. FS – 1.5LSB, FS – 0.5LSB).
The output is two’s complement binary with 1LSB =
FS – (– FS)/4096 = 5V/4096 = 1.22mV.
111...111
111...110
111...101
000...010
000...001
000...000
–(FS – 1LSB)
FS – 1LSB
INPUT RANGE
LTC1409 • F11a
Figure 11a. LTC1409 Transfer Characteristics
– 5V
R1
50k
R3
24k ANALOG INPUT
R4
100Ω
R5 R2
47k 50k
R6
24k
1
+A_IN
2
– A_IN
LTC1409
3
V_REF
4
REFCOMP
10µF
5
AGND
LTC1409 • F11b
Figure 11b. Offset and Full-Scale Adjust Circuit
In applications where absolute accuracy is important,
offset and full-scale errors can be adjusted to zero. Offset
error must be adjusted before full-scale error. Figure 11b
shows the extra components required for full-scale error
adjustment. Zero offset is achieved by adjusting the offset
applied to the – A_IN input. For zero offset error apply –
0.61mV (i.e., – 0.5LSB) at +A_IN and adjust the offset at the
– A_IN input until the output code flickers between 0000
0000 0000 and 1111 1111 1111. For full-scale adjust-
ment, an input voltage of 2.49817V (FS/2 – 1.5LSBs) is
applied to A_IN and R2 is adjusted until the output code
flickers between 0111 1111 1110 and 0111 1111 1111.
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