Datasheet

LTC2302/LTC2306
15
23026fa
APPLICATIONS INFORMATION
Figure 3a. Optional RC Input Filtering for Single-Ended Input
Figure 3b. Optional RC Input Filtering for Differential Inputs
Figure 4. 1kHz Sine Wave 8192 Point FFT Plot (LTC2306)
Figures 3a and 3b show respective examples of input
fi ltering for single-ended and differential inputs. For the
single-ended case in Figure 3a, a 50Ω source resistor
and a 2000pF capacitor to ground on the input will limit
the input bandwidth to 1.6MHz. High quality capacitors
and resistors should be used in the RC fi lter since these
components can add distortion. NPO and silver mica type
dielectric capacitors have excellent linearity. Carbon surface
mount resistors can generate distortion from self heating
and from damage that may occur during soldering. Metal
fi lm surface mount resistors are much less susceptible
to both problems.
Dynamic Performance
FFT (fast fourier transform) test techniques are used to test
the ADC’s frequency response, distortion and noise at the
rated throughput. By applying a low distortion sine wave
and analyzing the digital output using an FFT algorithm, the
ADC’s spectral content can be examined for frequencies
outside the fundamental.
Signal-to-Noise and Distortion Ratio (SINAD)
The signal-to-noise and distortion ratio (SINAD) is the
ratio between the RMS amplitude of the fundamental input
frequency to the RMS amplitude of all other frequency
components at the A/D output. The output is band-limited
to frequencies from above DC and below half the sampling
frequency. Figure 4 shows a typical SINAD of 72.8dB with
a 500kHz sampling rate and a 1kHz input. A SNR of 73.2dB
can be achieved with the LTC2302/LTC2306.
23026 F03a
CH0, CH1
LTC2306
V
REF
2000pF
10µF
0.1µF
0.1µF
50
ANALOG
INPUT
LT1790A-4.096
V
OUT
V
IN
5V
1000pF
23026 F03b
CH0, IN
+
CH1, IN
–
LTC2302
LTC2306
V
REF
1000pF
1000pF
10µF 0.1µF
50
50
DIFFERENTIAL
ANALOG
INPUTS
0.1µF
LT1790A-4.096
V
OUT
V
IN
5V
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.
FREQUENCY (kHz)
0
–40
–20
0
200
23026 F04
–60
–80
50 100 150 250
–100
–120
–50
–30
–10
–70
–90
–110
–130
–140
MAGNITUDE (dB)
SNR = 73.2dB
SINAD = 72.8dB
THD = –88.7dB