LTC2309 Linear Technology Corporation, LTC2309 Datasheet - Page 11

• Manufacturer Part Number: LTC2309
• Description: 12-Bit SAR ADC
• Manufacturer: Linear Technology Corporation

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Figures 4a and 4b show 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
APPLICATIONS INFORMATION
When using a fi lter with a large C
the inputs do not completely settle and the capacitive
input switching currents are averaged into a net DC
current (I
eled by an equivalent resistance (R
in series with an ideal voltage source (V
shown in Figure 3b. The magnitude of the DC current
is then approximately I
which is roughly proportional to V
DC drops across the resistor R
resistor and large capacitor should be chosen. When
running at the maximum throughput rate of 14ksps,
the input current equals 1.5μA at V
amounts to a full-scale error of 0.5LSB when using a
fi lter resistor (R
lower sample rates can tolerate a larger fi lter resistor
for the same amount of full-scale error.
V
V
IN
IN
DC
Figure 3a. Analog Input Equivalent Circuit
R
R
SOURCE
FILTER
). In this case, the analog input can be mod-
Figure 3b. Analog Input Equivalent
Circuit for Large Filter Capacitances
FILTER
I
DC
C
C
FILTER
FILTER
CH0-CH7
CH0-CH7
) of 333Ω. Applications requiring
INPUT
INPUT
DC
= (V
FILTER
IN
+
–
FILTER
R
1/(f
– V
EQ
EQ
V
IN
100Ω
, a fi lter with a small
REFCOMP
R
IN
SMPL
ON
. To prevent large
= 1/(f
REFCOMP
= 4.096V, which
value (e.g. 1μF),
• C
REFCOMP
/2
IN
LTC2309
LTC2309
)
SMPL
C
55pF
IN
2309 F03a
2309 F03b
/2)/R
• C
/2) as
IN
EQ
))
,
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
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 fre-
quency 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 14kHz sampling rate and a
1kHz input. An SNR of 73.4dB can be achieved with
the LTC2309.
Figure 4a. Optional RC Input Filtering for Single-Ended Input
Figure 4b. Optional RC Input Filtering for Differential Inputs
DIFFERENTIAL
ANALOG
INPUT
ANALOG
INPUTS
50Ω
0.1μF
50Ω
50Ω
0.1μF
2000pF
10μF
1000pF
1000pF
1000pF
10μF
CH0
COM
REFCOMP
CH0
CH1
REFCOMP
LTC2309
LTC2309
LTC2309
2309 F04a
2309 F04b
11
2309f