LTC2392-16
13
239216fa
applications inFormation
INPUT DRIVE CIRCUITS
A low impedance source can directly drive the high imped-
ance inputs of the LTC2392-16 without gain error. A high
impedance source should be buffered to minimize settling
time during acquisition and to optimize the distortion
performance of the ADC.
For best performance, a buffer amplifier should be used to
drive the analog inputs of the LTC2392-16. The amplifier
provides low output impedance to allow for fast settling
of the analog signal during the acquisition phase. It also
provides isolation between the signal source and the ADC
inputs which draw a small current spike during acquisition.
Input Filtering
The noise and distortion of the buffer amplifier and other
circuitry must be considered since they add to the ADC
noise and distortion. Noisy input circuitry should be filtered
prior to the analog inputs to minimize noise. A simple
1-pole RC filter is sufficient for many applications.
Large filter RC time constants slow down the settling at
the analog inputs. It is important that the overall RC time
constants be short enough to allow the analog inputs to
completely settle to 16-bit resolution within the acquisi-
tion time (t
ACQ
).
High quality capacitors and resistors should be used in the
RC
filter since these components can add distortion. NPO
and silver mica type dielectric capacitors have excellent
linearity. Carbon surface mount resistors can generate
distortion from self heating and from damage that may
occur during soldering. Metal film surface mount resistors
are much less susceptible to both problems.
Single-to-Differential Conversion
For single-ended input signals, a single-ended-to-differ-
ential conversion circuit must be used to produce a differ-
ential signal at the ADC inputs. The LT
®
6350 ADC driver is
recommended for performing a single-ended-to-differential
conversion, as shown in Figure 4a. Its low noise and good
DC linearity allows the LTC2392-16 to meet full data sheet
specifications. An alternative solution using two op amps
is shown in Figure 4b. Using two LT1806 op amps, the
circuit achieves 94dB signal-to-noise ratio (SNR). For a
20kHz input signal, the input of the LTC2392-16 has been
bandwidth limited to about 25kHz.
ADC REFERENCE
A low noise, low temperature drift reference is critical to
achieving the full data sheet performance of the ADC. The
LTC2392-16 provides an excellent internal reference with
a ±20ppm/°C (max) temperature coefficient. For better
accuracy, an
external reference can be used.
The
high speed, low noise internal reference buffer is used
for both internal and external reference applications. It
cannot be bypassed.
Figure 4a. Recommended Single-Ended-to-Differential
Conversion Circuit Using the LT6350 ADC Driver
Figure 4b. Alternative Single-Ended-to-Differential
Conversion Circuit Using Tw o LT1806 Op Amps
249Ω
249Ω
301Ω
ANALOG
INPUT
0V TO 4.096V
COMMON
MODE
VOLTAGE
301Ω
0.013µF
LTC2392-16
239216 F04b
IN
+
IN
–
–
+
LT1806
–
+
LT1806
249Ω
ANALOG INPUT
0V TO 4.096V
SINGLE-ENDED-
TO-DIFFERENTIAL
DRIVER
249Ω
2200pF
LTC2392-16
239216 F04a
IN
+
IN
–
LT6350
