21
21754312fa
LTC2175-12/
LTC2174-12/LTC2173-12
applicaTions inForMaTion
Figure 8. Reference Circuit
Figure 9. Using an External 1.25V Reference
ground. For a 2V input range with an external reference,
apply a 1.25V reference voltage to SENSE (Figure 9).
The input range can be adjusted by applying a voltage to
SENSE that is between 0.625V and 1.30V. The input range
will then be 1.6 • V
SENSE
.
The reference is shared by all four ADC channels, so it is
not possible to independently adjust the input range of
individual channels.
The V
REF
, REFH and REFL pins should be bypassed as
shown in Figure 8. The 0.1µF capacitor between REFH
Figure 10. Equivalent Encode Input Circuit for
Differential Encode Mode
and REFL should be as close to the pins as possible (not
on the backside of the circuit board).
Encode Input
The signal quality of the encode inputs strongly affects
the A/D noise performance. The encode inputs should
be treated as analog signals—do not route them next to
digital traces on the circuit board. There are two modes
of operation for the encode inputs: the differential encode
mode (Figure 10), and the single-ended encode mode
(Figure 11).
V
REF
REFH
SENSE
TIE TO V
DD
FOR 2V RANGE;
TIE TO GND FOR 1V RANGE;
RANGE = 1.6 • V
SENSE
FOR
0.65V < V
SENSE
< 1.300V
1.25V
REFL
0.1µF
2.2µF
INTERNAL ADC
HIGH REFERENCE
BUFFER
5Ω
0.8x
DIFF AMP
INTERNAL ADC
LOW REFERENCE
1.25V BANDGAP
REFERENCE
0.625V
RANGE
DETECT
AND
CONTROL
1µF
0.1µF
0.1µF
217512 F08
LTC2175-12
SENSE
1.25V
EXTERNAL
REFERENCE
1µF
1µF
V
REF
217512 F09
LTC2175-12
V
DD
LTC2175-12
217512 F10
ENC
–
ENC
+
15k
V
DD
DIFFERENTIAL
COMPARATOR
30k
30k
ENC
+
ENC
–
217512 F11
0V
1.8V TO 3.3V
LTC2175-12
CMOS LOGIC
BUFFER
Figure 11. Equivalent Encode Input Circuit for
Single-Ended Encode Mode
