Datasheet

IN1
MUX
GND
SAMPLING
CAPACITOR
SW1
-
+
CONTROL
LOGIC
CHARGE
REDISTRIBUTION
DAC
SW2
IN2
V
A
2
IN1
MUX
GND
SAMPLING
CAPACITOR
SW1
-
+
CONTROL
LOGIC
CHARGE
REDISTRIBUTION
DAC
SW2
IN2
V
A
2
ADC122S101
SNAS286D –MARCH 2005–REVISED MARCH 2013 www.ti.com
APPLICATIONS INFORMATION
ADC122S101 OPERATION
The ADC122S101 is a successive-approximation analog-to-digital converter designed around a charge-
redistribution digital-to-analog converter. Simplified schematics of the ADC122S101 in both track and hold modes
are shown in Figure 49 and Figure 50, respectively. In Figure 49, the ADC122S101 is in track mode: switch SW1
connects the sampling capacitor to one of two analog input channels through the multiplexer, and SW2 balances
the comparator inputs. The ADC122S101 is in this state for the first three SCLK cycles after CS is brought low.
Figure 50 shows the ADC122S101 in hold mode: switch SW1 connects the sampling capacitor to ground,
maintaining the sampled voltage, and switch SW2 unbalances the comparator. The control logic then instructs
the charge-redistribution DAC to add fixed amounts of charge to the sampling capacitor until the comparator is
balanced. When the comparator is balanced, the digital word supplied to the DAC is the digital representation of
the analog input voltage. The ADC122S101 is in this state for the fourth through sixteenth SCLK cycles after CS
is brought low.
The time when CS is low is considered a serial frame. Each of these frames should contain an integer multiple of
16 SCLK cycles, during which time a conversion is performed and clocked out at the DOUT pin and data is
clocked into the DIN pin to indicate the multiplexer address for the next conversion.
Figure 49. ADC122S101 in Track Mode
Figure 50. ADC122S101 in Hold Mode
USING THE ADC122S101
An ADC122S101 timing diagram and a serial interface timing diagram for the ADC122S101 are shown in the
Timing Diagrams section. CS is chip select, which initiates conversions and frames the serial data transfers.
SCLK (serial clock) controls both the conversion process and the timing of serial data. DOUT is the serial data
output pin, where a conversion result is sent as a serial data stream, MSB first. Data to be written to the
ADC122S101's Control Register is placed at DIN, the serial data input pin. New data is written to DIN with each
conversion.
A serial frame is initiated on the falling edge of CS and ends on the rising edge of CS. Each frame must contain
an integer multiple of 16 rising SCLK edges. The ADC output data (DOUT) is in a high impedance state when
CS is high and is active when CS is low. Thus, CS acts as an output enable. Additionally, the device goes into a
power down state when CS is high and also between continuous conversion cycles.
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