Datasheet

LTC2378-20
13
237820f
For more information www.linear.com/LTC2378-20
–180
AMPLITUDE (dBFS)
–60
–40
–20
–80
–100
–120
–140
–160
0
237820 F11b
SNR = 100dB
THD = –110dB
SINAD = 99.7dB
SFDR = 113dB
FREQUENCY (kHz)
0 100 200 300 400 500
Figure 11b. 64k Point FFT Plot
with f
IN
= 2kHz for Circuit Shown
in Figure 11a
Figure 10. Input Swing of the LTC2378 with Gain
Compression Enabled
applicaTions inForMaTion
Digital Gain Compression
The LTC2378-20 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 SAR ADC driver to be powered
off of a single positive supply since each input swings
between 0.5V and 4.5V as shown in Figure 10. Needing
only one positive supply to power the SAR ADC driver
results in additional power savings for the entire system.
With DGC enabled, the LTC2378-20 can be driven by the
low power LTC6362 differential driver which is powered
from a single 5V supply. Figure 11a shows how to configure
the LTC6362 to accept a ±3.28V true bipolar single-ended
input signal and level shift the signal to the reduced input
range of the LTC2378-20 when digital gain compression
is enabled. When paired with the LTC6655-4.096 for the
reference, the entire signal chain solution can be powered
from a single 5V supply, minimizing
power consump-
tion and reducing complexity. As shown in the FFT of
Figure11b, the single 5V supply solution can achieve up
to 100dB of SNR.
DC Accuracy
Many driver circuits presented in this data sheet emphasize
AC performance (Distortion and Signal to Noise Ratio), and
the amplifiers are chosen accordingly. The very low level
of distortion is a direct consequence of the excellent INL of
the LTC2378-20, and this property can be exploited in DC
applications as well. Note that while the LT6362 and LT6203
are characterized by excellent AC specifications, their DC
specifications do not match those of the LTC2378-20. The
offset of these amplifiers, for example, is more than 500μV
under certain conditions. In contrast, the LTC2378-20 has
a guaranteed maximum offset error of 130µV (typical drift
±0.007ppm/°C), and a guaranteed maximum full-scale
error of 100ppm (typical drift ±0.05ppm/°C). Low drift
is important to maintain accuracy over wide temperature
ranges in a calibrated system.
Amplifiers have to be selected very carefully to provide a
20-bit accurate DC signal chain. A large-signal open-loop
gain of at least 126dB may be required to ensure 1ppm
linearity for amplifiers configured for a
gain of negative
1.
However, less gain is sufficient if the amplifier’s gain
characteristic is known to be (mostly) linear. An ampli-
fier’s offset versus signal level must be considered for
amplifiers configured as unity gain buffers. For example,
Figure 11a. LTC6362 Configured to Accept a ±3.28V Input Signal While Running from
a Single 5V Supply When Digital Gain Compression Is Enabled in the LTC2378-20
237820 F10
5V
4.5V
0.5V
0V
237820 F11a
1k
V
CM
V
–
4
5
6
V
+
3
8
1
2
1k
1k
35.7Ω
3300pF
35.7Ω
1k
1k
V
CM
1k
0.41V
3.69V
0.41V
3.69V
4.096V
5V
47µF
10µF
LTC2378-20
REF/DGC
IN
+
REF V
DD
2.5V
IN
–
LTC6655-4.096V
IN
V
OUT_S
V
OUT_F
–3.28V
3.28V
0V
6800pF
6800pF
–
+
LTC6362