LTC2379-18
APPLICATIONS INFORMATION
V
CM
LT6350
5V
0V
OUT1
4
5V
0V
R2 = 499Ω
R
R
ꢁIT
ꢁIT
200pF
8
1
+
–
LT6350
5V
0V
OUT1
OUT2
4
5
5V
0V
–
+
R
ꢁIT
R
ꢁIT
8
+
–
OUT2
5
10μF
R4 = 402Ω
R3 = 2k
2
5V
0V
–
+
+
–
V
= V /2
REF
CM
1
2
237918 F05a
10V
0V
–10V
R
ꢁI
= 2k
R1 = 499Ω
+
–
V
= V /2
REF
CM
Figure 5a. ꢀT6350 Converting a 0V-5V Single-Ended
Signal to a 5V Differential Input Signal
220pF
237918 F06a
Figure 6a. ꢀT6350 Converting a 10V Single-Ended Signal to
a 5V Differential Input Signal
0
SIR = 101dꢀ
–20
–40
THD = –111.5dꢀ
SꢁIAD = 100.8dꢀ
SFDR = 114.5dꢀ
0
SIR = 100.8dꢀ
–20
–40
THD = –99.3dꢀ
SꢁIAD = 97.8dꢀ
SFDR = 101.2dꢀ
–60
–80
–60
–100
–120
–140
–160
–180
–80
–100
–120
–140
–160
–180
0
100 200 300 400 500 600 700 800
FREQUEICY (kHz)
237918 F05b
0
100 200 300 400 500 600 700 800
Figure 5b. 32k Point FFT Plot with fIN = 2kHz
for Circuit Shown in Figure 5a
FREQUEICY (kHz)
237918 F06b
Figure 6b. 32k Point FFT Plot with fIN = 2kHz
for Circuit Shown in Figure 6a
R1, R2, R3 and R4 must be selected in relation to R to
ꢁI
achievethedesiredattenuationandtomaintainabalanced
input impedance in the first amplifier. Table 1 shows the
5V
LT6203
5V
0V
3
2
+
–
resulting SIR and THD for several values of R , R1, R2,
ꢁI
1
7
0V
R3 and R4 in this configuration. Figure 6b shows the re-
sulting FFT when using the LT6350 as shown in Figure 6a.
5V
0V
5V
0V
5
6
+
–
Table 1. SNR, THD vs RIN for 10V Single-Ended Input Signal.
R
R1
ꢂΩx
R2
ꢂΩx
R3
ꢂΩx
R4
ꢂΩx
SNR
ꢂdꢁx
THD
ꢂdꢁx
IN
ꢂΩx
237918 F07
2k
499
499
2k
402
2k
100.8
100.5
94.8
–99
–94
–96
Figure 7. ꢀT6203 ꢁuffering a Fully Differential Signal Source
10k
100k
2.49k
24.9k
2.49k
24.9k
10k
100k
20k
Digital Gain Compression
The LTC2379-18 offers a digital gain compression (DGC)
feature which defines the full-scale input swing to be be-
Fully Differential Inputs
tween 10% and 90% of the V analog input range. To
REF
To achieve the full distortion performance of the
LTC2379-18,alowdistortionfullydifferentialsignalsource
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 –120dꢀ.
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. Ieeding only one
237918fa
12
Linear Systems [ Linear Systems ]
