Datasheet
LTC2378-18
12
237818f
R1, R2, R3 and R4 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,
R3 and R4 in this configuration. Figure 6b shows the re-
sulting FFT when using the LT6350 as shown in Figure 6a.
Table 1. SNR, THD vs R
IN
for ±10V Single-Ended Input Signal.
R
IN
(Ω)
R1
(Ω)
R2
(Ω)
R3
(Ω)
R4
(Ω)
SNR
(dB)
THD
(dB)
2k 499 499 2k 402 101.3 –100
10k 2.49k 2.49k 10k 2k 101 –92
100k 24.9k 24.9k 100k 20k 98.9 –97
Fully Differential Inputs
To achieve the full distortion performance of the
LTC2378-18, a low distortion fully differential signal source
driven through the LT6203 configured as two unity gain
buffers as shown in Figure 7 can be used to get the full
data sheet THD specification of –124dB.
APPLICATIONS INFORMATION
LT6350
R1 = 499
R2 = 499
R3 = 2k
R4 = 402
V
CM
= V
REF
/2
V
CM
237818 F06a
OUT1
R
INT
R
INT
R
IN
= 2k
OUT2
8
4
5
2
1
+
–
+
–
–
+
220pF
10µF
200pF
0V
5V
–10V
10V
0V
0V
5V
Digital Gain Compression
The LTC2378-18 offers a digital gain compression (DGC)
feature which defines the full-scale input swing to be be-
tween 10% and 90% of the ±V
REF
analog input range. To
enable digital gain compression, bring the REF/DGC pin
low. This feature allows the LT6350 to be powered off of
a single +5.5V supply since each input swings between
0.5V and 4.5V as shown in Figure 8. Needing only one
Figure 6a. LT6350 Converting a ±10V Single-Ended Signal to
a ±5V Differential Input Signal
Figure 6b. 32k Point FFT Plot with f
IN
= 2kHz
for Circuit Shown in Figure 6a
Figure 7. LT6203 Buffering a Fully Differential Signal Source
–180
AMPLITUDE (dBFS)
–60
–40
–20
–80
–100
–120
–140
–160
0
237818 F06b
SNR = 101.3dB
THD = –100.4dB
SINAD = 98.3dB
SFDR = 102.6dB
FREQUENCY (kHz)
0 100 200 300 500400
LT6203
237818 F07
0V
5V
0V
5V
3
1
2
+
–
0V
5V
5
7
6
+
–
0V
5V
LT6350
V
CM
= V
REF
/2
237818 F05a
0V
5V
0V
5V
OUT1
R
INT
R
INT
OUT2
8
4
5
2
1
+
–
+
–
–
+
0V
5V
FREQUENCY (kHz)
0 100 200 300 400 500
–180
AMPLITUDE (dBFS)
–60
–40
–20
–80
–100
–120
–140
–160
0
237818 F05b
SNR = 101.7dB
THD = –108.4dB
SINAD = 101.1dB
SFDR = 108.8dB
Figure 5a. LT6350 Converting a 0V-5V Single-Ended
Signal to a ±5V Differential Input Signal
Figure 5b. 32k Point FFT Plot with f
IN
= 2kHz
for Circuit Shown in Figure 5a