LTC2217 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2217' PDF : 32 Pages View PDF
LTC2217
APPLICATIONS INFORMATION
The internal programmable gain amplifier provides the
internal reference voltage for the ADC. This amplifier has
very stringent settling requirements and therefore is not
accessible for external use.
The SENSE pin can be driven ±5% around the nominal 2.5V
or 1.25V external reference inputs. This adjustment range
can be used to trim the ADC gain error or other system
gain errors. When selecting the internal reference, the
SENSE pin should be tied to VDD as close to the converter
as possible. If the sense pin is driven externally it should
be bypassed to ground as close to the device as possible
with 1μF ceramic capacitor.
1.575V
VCM
2.2μF
3.3V
1μF
2 LTC1461-2.5 6
4
SENSE LTC2217
2.2μF
2217 F07
Figure 7. A 2.75V Range ADC with
an External 2.5V Reference
Driving the Encode Inputs
The noise performance of the LTC2217 can depend on
the encode signal quality as much as on the analog input.
The encode inputs are intended to be driven differentially,
primarily for noise immunity from common mode noise
sources. Each input is biased through a 6k resistor to a
1.6V bias. The bias resistors set the DC operating point
for transformer coupled drive circuits and can set the logic
threshold for single-ended drive circuits.
Any noise present on the encode signal will result in ad-
ditional aperture jitter that will be RMS summed with the
inherent ADC aperture jitter.
In applications where jitter is critical (high input frequen-
cies), take the following into consideration:
1. Differential drive should be used.
2. Use as large an amplitude possible. If using trans-
former coupling, use a higher turns ratio to increase the
amplitude.
3. If the ADC is clocked with a fixed-frequency sinusoidal
signal, filter the encode signal to reduce wideband
noise.
4. Balance the capacitance and series resistance at both
encode inputs such that any coupled noise will appear
at both inputs as common mode noise.
The encode inputs have a common mode range of 1.2V
to VDD. Each input may be driven from ground to VDD for
single-ended drive.
LTC2217
VDD
ENC+
ENC–
VDD
1.6V
6k
VDD
1.6V
6k
TO INTERNAL
ADC CLOCK
DRIVERS
2217 F08a
Figure 8a. Equivalent Encode Input Circuit
0.1μF T1
50Ω
0.1μF
50Ω
ENC+
100Ω
8.2pF
LTC2217
0.1μF
ENC–
T1 = MA/COM ETC1-1-13
RESISTORS AND CAPACITORS
ARE 0402 PACKAGE SIZE
Figure 8b. Balun-Driven Encode
2217 F08b
2217f
21
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