ISL2671286 Datasheet, PDF (11/15 Pages) Intersil Corporation – 12-Bit, 20kSPS SAR ADC

ISL2671286
Functional Description
The ISL2671286 is based on a successive approximation register
(SAR) architecture utilizing capacitive charge redistribution
digital-to-analog converters (DACs). Figure 26 shows a simplified
representation of the converter. During the acquisition phase
(ACQ), the differential input is stored on the sampling capacitors
(CS). The comparator is in a balanced state since the switch
across its inputs is closed. The signal is fully acquired after t_ACQ
has elapsed, and the switches then transition to the conversion
phase (CONV) so the stored voltage can be converted to digital
format. The comparator becomes unbalanced when the
differential switch opens and the input switches transition
(assuming that the stored voltage is not exactly at mid-scale).
The comparator output reflects whether the stored voltage is
above or below mid-scale, which sets the value of the MSB. The
SAR logic then forces the capacitive DACs to adjust up or down by
one-quarter of full-scale by switching in binarily weighted
capacitors. Again, the comparator output reflects whether the
stored voltage is above or below the new value and sets the value
of the next lowest bit. This process repeats until all 12 bits have
been resolved.
VIN+
VIN–
CONV
ACQ
ACQ
ACQ CONV
CONV
VREF
SAR
LOGIC
FIGURE 26. SAR ADC ARCHITECTURAL BLOCK DIAGRAM
Analog Input
The ISL2671286 features a pseudo-differential input with a
nominal full-scale range equal to the applied VREF voltage. The
negative input (VIN–) must be biased within 200mV of ground.
Modes of Operation
There are two possible modes of operation, which are controlled
by the CS/SHDN signal. When CS/SHDN is high (deasserted), the
ADC is in static mode. Conversely, when CS/SHDN is low
(asserted), the device is in dynamic mode. There is no minimum
or maximum number of SCLK cycles required to enter static
mode. This simplifies power management and allows the user to
easily optimize power dissipation versus throughput for various
application requirements.
DYNAMIC MODE
This mode is entered when a conversion result is desired by
asserting CS/SHDN. Figure 28 shows the general operation in
this mode. The conversion is initiated on the falling edge of
CS/SHDN (refer to “Serial Digital Interface” section). When
CS/SHDN is deasserted, the conversion is terminated, and DOUT
returns to a high-impedance state. Sixteen serial clock cycles are
required to complete the conversion and access the complete
conversion result. CS/SHDN may idle high until the next
conversion or idle low until sometime prior to the next
conversion. Once a data transfer is complete (DOUT has returned
to a high-impedance state), another conversion can be initiated
by again asserting CS/SHDN.
CSB
SCLK
1
10
16
ADC Transfer Function
The output coding for the ISL2671286 is straight binary. The first
code transition occurs at successive LSB values (i.e., 1 LSB, 2
LSB, and so on). The LSB size is VREF/4096. The ideal transfer
characteristic of the ISL2671286 is shown in Figure 27.
111...111
111...110
1LSB = VREF/4096
100...001
100...000
011...111
000...010
000...001
000...000
0V +½LSB
+VREF +VREF
– 1½LSB – 1LSB
ANALOG INPUT
+IN – (–IN)
FIGURE 27. IDEAL TRANSFER CHARACTERISTICS
DOUT
4 LEADING ZEROS AND CONVERSION RESULT
FIGURE 28. NORMAL MODE OPERATION
STANDBY MODE
The ISL2671286 enters the power-saving static mode
automatically any time CS/SHDN is deasserted. The user is not
required to force a device into this mode following a conversion
in order to optimize power consumption.
SHORT CYCLING
In cases where a lower resolution conversion is acceptable,
CS/SHDN can be pulled high before 12 SCLK falling edges have
elapsed. This is referred to as short cycling, and it can be used to
further optimize power dissipation. In this mode, a lower
resolution result is acquired, but the ADC enters static mode
sooner and exhibits a lower average power dissipation than if the
complete conversion cycle is carried out. The acquisition time
(t_ACQ) requirement must be met for the next conversion to be
valid.
POWER-ON RESET
The ISL2671286 performs a power-on reset that requires
approximately 2.5ms to execute when the supplies are first
11
FN7863.0
November 1, 2011