Datasheet

LTC2301/LTC2305
14
23015fb
APPLICATIONS INFORMATION
The magnitude of the DC current is then approximately
I
DC
= (V
IN
– V
REFCOMP
/2)/R
EQ
, which is roughly proportional
to V
IN
. To prevent large DC drops across the resistor R
FILTER
,
a fi lter with a small resistor and large capacitor should be
chosen. When running at the maximum throughput rate
of 14ksps, the input current equals 1.5µA at V
IN
= 4.096V,
which amounts to a full-scale error of 0.5 LSBs when using
a fi lter resistor (R
FILTER
) of 333Ω. Applications requiring
lower sample rates can tolerate a larger fi lter resistor for
the same amount of full-scale error.
Figures 3a and 3b 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
Figure 2a. Analog Input Equivalent Circuit
Figure 2b. Analog Input Equivalent
Circuit for Large Filter Capacitances
V
IN
INPUT
CH0, CH1,
IN
+
, IN
–
R
ON
=
100
C
IN
=
55pF
C
FILTER
R
SOURCE
23015 F02a
LTC2301
LTC2305
V
IN
INPUT
(CH0, CH1,
IN
+
, IN
–
)
R
EQ
=
1/(f
SMPL
• C
IN
)
V
REFCOMP
/2
C
FILTER
R
FILTER
I
DC
23015 F02b
LTC2301
LTC2305
+
–
and from damage that may occur during soldering. Metal
fi lm surface mount resistors are much less susceptible
to both problems.
Dynamic Performance
Fast Fourier Transform (FFT) 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
Figure 3a. Optional RC Input Filtering for Single-Ended Input
Figure 3b. Optional RC Input Filtering for Differential Inputs
23015 F03a
CH0, CH1
LTC2305
REFCOMP
2000pF
0.1µF
10µF
50
ANALOG
INPUT
1000pF
23015 F03b
CH0, IN
+
CH1, IN
–
LTC2301
LTC2305
REFCOMP
1000pF
1000pF
0.1µF10µF
50
50
DIFFERENTIAL
ANALOG
INPUTS