Datasheet

LTC2309
13
2309fd
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
V V V V
V
N
=
+ + +
20
2
2
3
2
4
2 2
1
log
...
where V
1
is the RMS amplitude of the fundamental
frequency and V
2
through V
N
are the amplitudes of the
second through Nth harmonics.
Internal Reference
The LTC2309 has an on-chip, temperature compen-
sated bandgap reference that is factory trimmed to
2.5V (Refer to Figure 6a). It is internally connected to a
reference amplifier and is available at V
REF
. V
REF
should
be bypassed to GND with a 2.2μF ceramic capacitor to
minimize noise. An 8k resistor is in series with the output
so that it can be easily overdriven by an external refer-
ence if more accuracy and/or lower drift are required,
as shown in Figure 6b. The reference amplifier gains
the V
REF
voltage by 1.638 to 4.096V at REFCOMP. To
compensate the reference amplifier, bypass REFCOMP
with a 10μF ceramic capacitor in parallel with a 0.1μF
ceramic capacitor for best noise performance. The
internal reference buffer can also be overdriven from
1V to V
DD
, as shown in Figure 6c. To do so, V
REF
must
be grounded to disable the reference buffer.
AppLICAtIOns InFORMAtIOn
Figure 5. 1kHz Sine Wave 8192 Point FFT Plot
R2
R3
REFERENCE
AMP
0.1µF
10µF
2.2µF
REFCOMP
GND
V
REF
R1
8k
2.5V
4.096V
LTC2309
2309 F06a
BANDGAP
REFERENCE
Figure 6a. LTC2309 Reference Circuit
0.1µF
10µF
2309 F06b
LT1790A-2.5
V
OUT
V
IN
5V
V
REF
LTC2309
GND
REFCOMP
2.2µF
0.1µF
Figure 6b. Using the LT
®
1790A-2.5 as an External Reference
FREQUENCY (kHz)
0
–140
MAGNITUDE (dB)
–120
–100
–80
0
–40
1
3
4 7
2309 G03
–20
–60
2
5
6
SNR = 73.4dB
SINAD = 73.3dB
THD = –88dB
Figure 6c. Overdriving REFCOMP Using the LT1790A-4.096
0.1µF
0.1µF
10µF
2309 F06c
LT1790A-4.096
V
OUT
V
IN
5V
V
REF
LTC2309
GND
REFCOMP