LTC2173-14 Linear Technology Corporation, LTC2173-14 Datasheet - Page 21

• Manufacturer Part Number: LTC2173-14
• Description: (LTC217x-14) 125Msps Low Power Quad ADCs
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
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
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
shown in Figure 8. The 0.1μF capacitor between REFH
TIE TO GND FOR 1V RANGE;
TIE TO V
RANGE = 1.6 • V
0.65V < V
REF
DD
2.2μF
FOR 2V RANGE;
Figure 9. Using an External 1.25V Reference
SENSE
0.1μF
0.1μF
, REFH and REFL pins should be bypassed as
1.25V
SENSE
< 1.300V
REFERENCE
EXTERNAL
FOR
Figure 8. Reference Circuit
1.25V
SENSE
0.1μF
1μF
SENSE
REFH
V
REFL
REF
.
LTC2175-14
SENSE
1μF
1μF
CONTROL
5Ω
DETECT
V
RANGE
REF
AND
LTC2175-14
1.25V BANDGAP
DIFF AMP
REFERENCE
INTERNAL ADC
HIGH REFERENCE
0.8x
INTERNAL ADC
LOW REFERENCE
217514 F09
0.625V
BUFFER
217514 F08
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).
ENC
ENC
Figure 11. Equivalent Encode Input Circuit for
Single-Ended Encode Mode
LTC2174-14/LTC2173-14
Figure 10. Equivalent Encode Input Circuit for
Differential Encode Mode
+
–
1.8V TO 3.3V
LTC2175-14
15k
30k
0V
V
DD
V
ENC
ENC
DD
+
–
LTC2175-14
LTC2175-14/
30k
DIFFERENTIAL
COMPARATOR
CMOS LOGIC
BUFFER
217514 F11
217514 F10
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