Datasheet

LTC2203/LTC2202
21
22032fd
Figure 5 demonstrates the use of an LTC1994 differential
amplifi er to convert a single ended input signal into a
differential input signal. The advantage of this method is
that it provides low frequency input response; however,
the limited gain bandwidth of any op amp will limit the
SFDR at high input frequencies.
The internal programmable gain amplifi er provides the
internal reference voltage for the ADC. This amplifi er has
very stringent settling requirements and is not accessible
for external use.
Figure 5. DC Coupled Input with Differential Amplifi er
V
CM
22032 F05
+
+
CM LT1994
499Ω
100pF
100pF
100pF
2.2 μF
499Ω
523Ω
25Ω
25Ω
499Ω
53.6Ω
A
IN
+
A
IN
LTC2203/
LTC2202
Figure 6. Reference Circuit
PGA
1.25V
SENSE
V
CM
BUFFER
INTERNAL
ADC
REFERENCE
RANGE
SELECT
AND GAIN
CONTROL
2.5V
BANDGAP
REFERENCE
2.2μF
TIE TO V
DD
TO USE
INTERNAL 2.5V
REFERENCE
OR INPUT FOR
EXTERNAL 2.5V
REFERENCE
OR INPUT FOR
EXTERNAL 1.25V
REFERENCE
22032 F07
LTC2203/LTC2202
Figure 7. A 2.25V Range ADC with
an External 2.5V Reference
V
CM
SENSE
1.25V
3.3V
2.2μF
2.2μF
F
22032 F08
LT1461-2.5
2
6
4
LTC2203/
LTC2202
The SENSE pin can be driven ±5% around the nominal 2.5V
or 1.25V external reference input. 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 V
DD
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 at least a 1μF ceramic capacitor.
APPLICATIONS INFORMATION
The 25Ω resistors and 12pF capacitor on the analog
inputs serve two purposes: isolating the drive circuitry
from the sample-and-hold charging glitches and limiting
the wideband noise at the converter input.
Reference Operation
Figure 6 shows the LTC2203/LTC2202 reference circuitry
consisting of a 2.5V bandgap reference, a programmable
gain amplifi er and control circuit. The LTC2203/LTC2202
has three modes of reference operation: Internal Refer-
ence, 1.25V external reference or 2.5V external reference.
To use the internal reference, tie the SENSE pin to V
DD
. To
use the external reference, simply apply either a 1.25V or
2.5V reference voltage to the SENSE input pin. Both 1.25V
and 2.5V applied to SENSE will result in a full scale range
of 2.5V
P-P
(PGA = 0). A 1.25V output, V
CM
, is provided
for a common mode bias for input drive circuitry. An
external bypass capacitor is required for the V
CM
output.
This provides a high frequency low impedance path to
ground for internal and external circuitry. This is also the
compensation capacitor for the reference; it will not be
stable without this capacitor. The minimum value required
for stability is 2.2μF.