ltc2203 Linear Technology Corporation, ltc2203 Datasheet - Page 21

• Manufacturer Part Number: ltc2203
• Description: 16-bit, 25msps Adc
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
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 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
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
for a common mode bias for input drive circuitry. An
external bypass capacitor is required for the V
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.
2.2 μF
Figure 5. DC Coupled Input with Differential Amplifi er
P-P
53.6Ω
(PGA = 0). A 1.25V output, V
523Ω
499Ω
U
–
CM LT1994
+
499Ω
499Ω
+
–
U
100pF
100pF
100pF
W
CM
V
A
A
CM
IN
IN
–
, is provided
+
CM
U
LTC2203/02
output.
DD
22032 F05
. To
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.
TIE TO V
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
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.
EXTERNAL 1.25V
EXTERNAL 2.5V
INTERNAL 2.5V
OR INPUT FOR
OR INPUT FOR
REFERENCE
REFERENCE
REFERENCE
DD
TO USE
3.3V
1μF
Figure 7. A 2.25V Range ADC with
an External 2.5V Reference
SENSE
2.2μF
V
CM
Figure 6. Reference Circuit
2
LT1461-2.5
LTC2203/02
1.25V
LTC2203/LTC2202
4
BUFFER
6
DD
AND GAIN
CONTROL
SELECT
RANGE
as close to the converter
SENSE
2.2μF
2.2μF
V
CM
1.25V
LTC2203/02
PGA
22032 F08
REFERENCE
BANDGAP
REFERENCE
INTERNAL
2.5V
21
ADC
22032fb
22032 F07
S