ADC121C021CIMKX NSC [National Semiconductor], ADC121C021CIMKX Datasheet - Page 14

• Manufacturer Part Number: ADC121C021CIMKX
• Description: I2C-Compatible, 12-Bit Analog-to-Digital Converter (ADC) with Alert Function
• Manufacturer: NSC [National Semiconductor]

www.national.com
1.0 Functional Description
The ADC121C021 is a successive-approximation analog-to-
digital converter designed around a charge-redistribution dig-
ital-to-analog converter. Unless otherwise stated, references
to the ADC121C021 in this section will apply to both the
ADC121C021 and the ADC121C027.
1.1 CONVERTER OPERATION
Simplified schematics of the ADC121C021 in both track and
hold operation are shown in Figure 2 and Figure 3 respec-
tively. In Figure 2, the ADC121C021 is in track mode; switch
SW1 connects the sampling capacitor to the analog input
channel, and SW2 equalizes the comparator inputs. The ADC
is in this state for approximately 0.4µs at the beginning of ev-
ery conversion cycle. Conversions occur when the conver-
sion result register is read by the I
ADC is in automatic conversion mode. (see Section 1.9)
Figure 3 shows the ADC121C021 in hold mode: switch SW1
connects the sampling capacitor to ground and switch SW2
unbalances the comparator. The control logic then instructs
the charge-redistribution DAC to add or subtract fixed
amounts of charge to or from the sampling capacitor until the
comparator is balanced. When the comparator is balanced,
the digital word supplied to the DAC is also the digital repre-
sentation of the analog input voltage. This digital word is
stored in the conversion result register and read via the 2-wire
interface.
FIGURE 2. ADC121C021 in Track Mode
FIGURE 3. ADC121C021 in Hold Mode
2
C controller and when the
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1.2 ANALOG INPUT
An equivalent circuit for the input of the ADC121C021 is
shown in Figure 4. Diodes D1 and D2 provide ESD protection
for the analog input. The operating range for the analog input
is 0 V to V
diodes to conduct and result in erratic operation.
The capacitor C1 in Figure 4 has a typical value of 3 pF and
is mainly the package pin capacitance. Resistor R1 is the on
resistance (R
is typically 500Ω. Capacitor C2 is the ADC121C021 sampling
capacitor, and is typically 30 pF. The ADC121C021 will de-
liver best performance when driven by a low-impedance
source (less than 100Ω). This is especially important when
using the ADC121C021 to sample dynamic signals. The dy-
namic performance of the ADC will be affected significantly
by large source impedances. An input buffer amplifier may be
necessary to limit source impedance. A high-accuracy op-
amp is recommended to maximize circuit performance. Also
important when sampling dynamic signals is an anti-aliasing
band-pass or low-pass filter which reduces harmonics and
noise at the input.
A
FIGURE 4. Equivalent Input Circuit
. Going beyond this range will cause the ESD
ON
) of the multiplexer and track / hold switch and
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