AS1545_1 AMSCO [austriamicrosystems AG], AS1545_1 Datasheet - Page 19

• Manufacturer Part Number: AS1545_1
• Description: Dual, 12-Bit, 1MSPS, SAR ADC
• Manufacturer: AMSCO [austriamicrosystems AG]

AS1545
Datasheet - D e t a i l e d D e s c r i p t i o n
Figure 34. Typical Application
Analog Input Composition
The equivalent circuit of analog input structure of the AS1545 in differential/pseudo differential modes is shown in the
Figure
analog inputs. These diodes can conduct up to 10mA without causing irreversible damage to the part.
Note: Make sure that the analog input signals never exceed the supply rails by more than 300mV. This causes the
The C1 capacitors in
typically about 100Ω. The C2 capacitors are the ADC’s sampling capacitors with a capacitance of 20pF typically.
For ac applications, removing high frequency components from the analog input signal is recommended by the use of
an RC low-pass filter on the relevant analog input pins with optimum values of 10Ω and 10nF. In applications where
harmonic distortion and signal-to-noise ratio are critical, the analog input should be driven from a low impedance
source. Large source impedances significantly affect the ac performance of the ADC and may necessitate the use of
an input buffer amplifier. The choice of the op amp is a function of the particular application.
Figure 35. Equivalent Analog Input Circuit
When no amplifier is used to drive the analog input, the source impedance should be limited to low values. The
maximum source impedance depends on the amount of THD that can be tolerated. The THD increases as the source
impedance increases and performance degrades.
Figure 23 on page 13
ended mode, while
mode.
Figure 22 on page 13
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AIN+
35. In single-ended mode, AIN- is internally tied to AGND. The four diodes provide ESD protection for the
diodes to become forward-biased and starts conducting into the substrate.
C1
4pF
VDD
Figure 25
Figure 35
+
-
+
-
shows a graph of the THD vs. the analog input signal frequency for various supplies in single-
shows a graph of the THD vs. the analog input frequency for different source impedances.
D
D
Open for conversion,
closed for tracking.
shows the THD vs. the analog input signal frequency for various supplies in differential
are of 4pF and can be attributed to pin capacitance. The value of these resistors is
10Ω
10Ω
10Ω
10Ω
R1
10nF
AIN 1
10nF
10nF
BIN 1
10nF
AIN 0
BIN 0
C2
AVDD
REFSEL
Revision 1.01
10µF
1µF
100nF
AS1545
A0 A1 A2
DVDD
AIN-
SGL/DIFF
VDRIVE
RANGE
DOUTB
DOUTA
100nF
REFB
REFA
C1
SCLK
10µF
CSN
1µF
4pF
4.7µF
1µF
VDD
D
D
100nF
4.7µF
Open for conversion,
closed for tracking.
µC
R1
C2
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