12
LTC1609
1609fa
APPLICATIO S I FOR ATIO
WUUU
Table 1c. LTC1609 Output Codes for Ideal Input Voltages
TWO’S COMPLEMENT STRAIGHT BINARY
DESCRIPTION ANALOG INPUT (SB/BTC LOW) (SB/BTC HIGH)
Full-Scale Range ±10V ±5V ±3.34 0V to 10V 0V to 5V 0V to 4V
Least Significant Bit 305µV 153µV 102µV 153µV76µV61µV
+Full Scale (FS – 1LSB) 9.999695V 4.999847V 3.339898V 9.999847V 4.999924V 3.999939V 0111 1111 1111 1111 1111 1111 1111 1111
Midscale 0V 0V 0V 5V 2.5V 2V 0000 0000 0000 0000 1000 0000 0000 0000
1LSB Below Midscale –305µV –153µV –102µV 4.999847V 2.499924V 1.999939V 1111 1111 1111 1111 0111 1111 1111 1111
–Full Scale –10V –5V –3.340000V 0V 0V 0V 1000 0000 0000 0000 0000 0000 0000 0000
INPUT VOLTAGE (V)
0V
OUTPUT CODE (TWO’S COMPLIMENT)
–1
LSB
1609 F04a
011...111
011...110
000...001
000...000
100...000
100...001
111...110
1
LSB
BIPOLAR
ZERO
111...111
FSR/2 – 1LSB–FSR/2
FSR = +FS – –FS
1LSB = FSR/65536
INPUT VOLTAGE (V)
OUTPUT CODE
1609 F04b
111...111
111...110
100...001
100...000
000...000
000...001
011...110
011...111
FS – 1LSB0V
1LSB = FS/65536
Figure 4a. LTC1609 Bipolar Transfer Characteristics Figure 4b. LTC1609 Unipolar Transfer Characteristics
The ideal ±FS value for the ±3.3V range is 3.340000V – 1LSB
and – 3.340000V, respectively. The external 33.2k resistor
that is connected between the CAP pin and the R2
IN
pin,
slightly attenuates the input signal applied to R2
IN
. With-
out the 33.2k resistor the ±FS value would be 3.333333V
– 1LSB and –3.333333V (zero volt offset), respectively.
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 conver-
sions. For example in Figure 5 the distribution of output
code is shown for a DC input that has been digitized 4096
times. The distribution is Gaussian and the RMS code
transition is about 0.9LSB.
Figure 5. Histogram for 4096 Conversions
CODE
–3
0
COUNT
500
1000
1500
2000
–2 –1 0 1
1609 F05
23
