Datasheet

LTC2381-16
11
238116f
APPLICATIONS INFORMATION
Single-Ended-to-Differential Conversion
For single-ended input signals, a single-ended to differential
conversion circuit must be used to produce a differential
signal at the inputs of the LTC2381-16. The LT6350 ADC
driver is recommended for performing single-ended-to-
differential conversions.The LT6350 is flexible and may
be configured to convert single-ended signals of various
amplitudes to the ±2.5V differential input range of the
LTC2381-16. The LT6350 is also available in H-grade to
complement the extended temperature operation of the
LTC2381-16 up to 125°C.
Figure 5 shows the LT6350 being used to convert a 0V
to 2.5V single-ended input signal. In this case, the first
amplifier is configured as a unity gain buffer and the single-
ended input signal directly drives the high-impedance
input of the amplifier. As shown in the FFT of Figure 5a,
the LT6350 drives the LTC2381-16 to full datasheet
performance without degrading the SNR or THD .
The LT6350 can also be used to buffer and convert
single-ended signals larger than the input range of the
LTC2381-16 in order to maximize the signal swing that
can be digitized. Figure 6 shows the LT6350 converting a
0V-5V single-ended input signal to the ±2.5V differential
input range of the LTC2381-16. In this case, the first
amplifier in the LT6350 is configured as an inverting
amplifier stage, which acts to attenuate the input signal
down to the 0V-2.5V input range of the LTC2381-16. In the
inverting amplifier configuration, the single-ended input
signal source no longer directly drives a high impedance
input of the first amplifier. The input impedance is instead
set by resistor R
IN
. R
IN
must be chosen carefully based on
the source impedance of the signal source. Higher values
of R
IN
tend to degrade both the noise and distortion of
the LT6350 and LTC2381-16 as a system. R1, R2 and R3
must be selected in relation to R
IN
to achieve the desired
attenuation and to maintain a balanced input impedance
in the first amplifier. Table 1 shows the resulting SNR
and THD for several values of R
IN
, R1, R2 and R3 in this
configuration. Figure 6a shows the resulting FFT when
using the LT6350 as shown in Figure 6.
The LT6350 can also be used to buffer and convert large,
true bipolar signals which swing below ground to the ±2.5V
differential input range of the LTC2381-16. Figure 7 shows
the LT6350 being used to convert a ±10V true bipolar signal
for use by the LTC2381-16. The input impedance is again
set by resistor R
IN
. Table 2 shows the resulting SNR and
THD for several values of R
IN
. Figure 7a shows the resulting
FFT when using the LT6350 as shown in Figure 7.
LT6350
V
CM
= V
REF
/2
2.5V to
0V
0V to
2.5V
0V to 2.5V
238116 F05
OUT1
R
INT
R
INT
OUT2
8
4
5
2
1
+
–
+
–
–
+
FREQUENCY (kHz)
0 25 50 75 100 125
–180
AMPLITUDE (dBFS)
–60
–40
–20
–80
–100
–120
–140
–160
0
238116 F05a
SNR = 91.8dB
THD = –106dB
SINAD = 91.6dB
SFDR = 107dB
Figure 5. LT6350 Converting a 0V-2.5V Single-Ended Signal
to a ±2.5V Differential Input Signal
Figure 5a. 32k Point FFT Plot for Circuit Shown in Figure 5