LTC2309_15 Datasheet, PDF (12/26 Pages) Linear Technology – 8-Channel, 12-Bit SAR ADC with IC Interface

LTC2309
Applications Information
When using a filter with a large C_FILTER value (e.g. 1µF),
the inputs do not completely settle and the capacitive
input switching currents are averaged into a net DC
current (I_DC). In this case, the analog input can be mod-
eled by an equivalent resistance (R_EQ = 1/(f_SMPL • C_IN))
in series with an ideal voltage source (V_REFCOMP/2), as
shown in Figure 3b. 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 filter 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 0.5LSB when using a
filter resistor (R_FILTER) of 333Ω. Applications requiring
lower sample rates can tolerate a larger filter resistor
for the same amount of full-scale error.
R_SOURCE
V_IN
INPUT
CH0-CH7
C_FILTER
R_ON
100Ω
LTC2309
C_IN
55pF
2309 F03a
Figure 3a. Analog Input Equivalent Circuit
R_FILTER
V_IN
INPUT
I_{DC CH0-CH7}
C_FILTER
R_EQ
LTC2309
1/(f_SMPL • C_IN)
+– _VREFCOMP/2
2309 F03b
Figure 3b. Analog Input Equivalent
Circuit for Large Filter Capacitances
Figures 4a and 4b show examples of input filtering 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 filter since these
components can add distortion. NPO and silver mica
type dielectric capacitors have excellent linearity. Carbon
surface mount resistors can generate distortion from
12
ANALOG
50Ω
INPUT
CH0
2000pF
LTC2309
COM
0.1µF
10µF
REFCOMP
2309 F04a
Figure 4a. Optional RC Input Filtering for Single-Ended Input
50Ω
DIFFERENTIAL
ANALOG
INPUTS
50Ω
0.1µF
1000pF
CH0
1000pF
LTC2309
CH1
1000pF
10µF
REFCOMP
2309 F04b
Figure 4b. Optional RC Input Filtering for Differential Inputs
self heating and from damage that may occur during
soldering. Metal film 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.
2309fd