Datasheet

LTC2308
12
2308fb
APPLICATIONS INFORMATION
Figure 3b. Analog Input Equivalent Circuit
for Large Filter Capacitances
Figure 4b. Optional RC Input Filtering for Differential Inputs
2308 F04a
CH0
COM
LTC2308
REFCOMP
2000pF
10μF 0.1μF
50Ω
ANALOG
INPUT
1000pF
2308 F04b
CH0
CH1
LTC2308
REFCOMP
1000pF
1000pF
10μF 0.1μF
50Ω
50Ω
DIFFERENTIAL
ANALOG
INPUTS
time of 2μs, the input current equals 106μA at V
IN
= 5V,
which amounts to a full-scale error of 0.5LSBs when using
a fi lter resistor (R
FILTER
) of 4.7Ω. Applications requiring
lower sample rates can tolerate a larger fi lter resistor for
the same amount of full-scale error.
Figures 4a and 4b show respective examples of input
fi ltering for single-ended and differential inputs. For the
single-ended case in Figure 4a, 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.
V
IN
INPUT
CH0-CH7
R
ON
100Ω
C
IN
55pF
C1
R
SOURCE
2308 F03a
LTC2308
V
IN
INPUT
CH0-CH7
V
REFCOMP
/2
R
EQ
1/(f
SMPL
• C
IN
)
C
FILTER
R
FILTER
I
DC
2308 F03b
LTC2308
+
–
Figure 4a. Optional RC Input Filtering for Single-Ended Input
Figure 3a. Analog Input Equivalent Circuit
Figure 5. 1kHz Sine Wave 8192 Point FFT Plot
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 5 shows a typical SINAD of 73.3dB with
a 500kHz sampling rate and a 1kHz input. A SNR of 73.4dB
can be achieved with the LTC2308.
FREQUENCY (kHz)
0
–140
MAGNITUDE (dB)
–120
–100
–80
0
–40
100
150 250
–20
–60
–130
–110
–90
–10
–50
–30
–70
50
200
2308 TA01b