Datasheet

LTC2308
13
2308fb
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(f
SMPL
/2).
THD is expressed as:
THD
VVV V
V
N
=
++ +
20
2
2
3
2
4
22
1
log
...
where V
1
is the RMS amplitude of the fundamental fre-
quency and V
2
through V
N
are the amplitudes of the second
through Nth harmonics.
Internal Reference
The LTC2308 has an on-chip, temperature compensated
bandgap reference that is factory trimmed to 2.5V (Refer
to Figure 6a). 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 2.2μF tantalum 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 6b. The reference amplifi er gains the
V
REF
voltage by 1.638 to 4.096V at REFCOMP (Pin 8). To
compensate the reference amplifi er, bypass REFCOMP
with a 10μF ceramic or tantalum capacitor in parallel with
a 0.1μF ceramic capacitor for best noise performance. The
internal reference buffer can also be overdriven from 1V
to AV
DD
with an external reference at REFCOMP as shown
in Figure 6c. 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.
Internal Conversion Clock
The internal conversion clock is factory trimmed to
achieve a typical conversion time (t
CONV
) of 1.3μs and a
maximum conversion time of 1.6μs over the full operat-
ing temperature range. With a typical acquisition time of
240ns, a throughput sampling rate of 500ksps is tested
and guaranteed.
APPLICATIONS INFORMATION
Figure 6a. LTC2308 Reference Circuit
Figure 6b. Using the LT1790A-2.5 as an External Reference
R2
R3
REFERENCE
AMP
0.1μF
10μF
2.2μF
REFCOMP
GND
V
REF
R1
8k
2.5V
4.096V
LTC2308
2308 F06a
BANDGAP
REFERENCE
0.1μF
10μF
2308 F06b
LT1790A-2.5
V
OUT
V
IN
5V
V
REF
LTC2308
GND
REFCOMP
+
2.2μF
0.1MF
0.1μF
10μF
2308 F06c
LT1790A-4.096
V
OUT
V
IN
5V
V
REF
LTC2308
GND
REFCOMP
+
Figure 6c. Overdriving REFCOMP Using the LT1790A-4.096
Digital Interface
The LTC2308 communicates via a standard 4-wire SPI
compatible digital interface. The rising edge of CONVST
initiates a conversion. After the conversion is fi nished, pull
CONVST low to enable the serial output (SDO). The ADC
shifts out the digital data in 2’s complement format when
operating in bipolar mode or in straight binary format when
in unipolar mode, based on the setting of the UNI bit.