Datasheet

LTC2301/LTC2305
15
23015fb
APPLICATIONS INFORMATION
frequency. Figure 4 shows a typical SINAD of 73.2dB with
a 14kHz sampling rate and a 1kHz input. A SNR of 73.3dB
can be achieved with the LTC2301/LTC2305.
Figure 4. 1kHz Sine Wave 8192 Point FFT Plot
Total Harmonic Distortion (THD)
Total Harmonic Distortion (THD) is the ratio of the RMS sum
of all harmonics of the input signal to the fundamental itself.
The out-of-band harmonics alias into the frequency band
between DC and half the sampling frequency (
fSMPL
/2).
THD is expressed as:
THD = 20log
V
2
2
+ V
3
2
+ V
4
2
...+ V
N
2
V
1
where V1 is the RMS amplitude of the fundamental fre-
quency and V2 through V
N
are the amplitudes of the second
through Nth harmonics.
Internal Reference
The LTC2301/LTC2305 have an on-chip, temperature
compensated bandgap reference that is factory trimmed
to 2.5V (Refer to Figure 5a). It is internally connected to
a reference amplifi er and is available at V
REF
(Pin 7). V
REF
should be bypassed to GND with a 2.2F ceramic capacitor
to minimize noise. An 8k resistor is in series with the output
so that it can be easily overdriven by an external reference
if more accuracy and/or lower drift are required, as shown
in Figure 5b. The reference amplifi er gains the V
REF
volt-
age by 1.638 to 4.096V at REFCOMP. To compensate the
reference amplifi er, bypass REFCOMP with a 10F ceramic
capacitor in parallel with a 0.1F ceramic capacitor for best
noise performance. The internal reference buffer can also
be overdriven from 1V to V
DD
with an external reference at
REFCOMP, as shown in Figure 5c. To do so, V
REF
must be
grounded to disable the reference buffer. This will result
in an input range of 0V to V
REFCOMP
in unipolar mode and
±0.5 • V
REFCOMP
in bipolar mode.
R2
R3
REFERENCE
AMP
0.1µF
10µF
2.2µF
REFCOMP
GND
V
REF
R1
8k
2.5V
4.096V
LTC2301
LTC2305
23015 F05a
BANDGAP
REFERENCE
Figure 5a. LTC2301/LTC2305 Reference Circuit
Figure 5b. Using the LT1790A-2.5 as an External Reference
Figure 5c. Overdriving REFCOMP Using the LT1790A-4.096
0.1µF
10µF
23015 F05b
LT1790A-2.5
V
OUT
V
IN
5V
V
REF
LTC2301
LTC2305
GND
REFCOMP
2.2µF
0.1µF
0.1µF
10µF
23015 F05c
LT1790A-4.096
V
OUT
V
IN
5V
V
REF
LTC2301
LTC2305
GND
REFCOMP
0.1µF
FREQUENCY (kHz)
0
MAGNITUDE (dB)
0
–70
–60
–50
–40
–30
–20
–10
–90
–110
–130
–80
–100
–120
–140
51234 6
23015 F04
7
SNR = 73.2 dB
SINAD = 73.1 dB
THD = –90dB