
AD5590
Rev. A | Page 27 of 44
ADC SECTION
The ADC section is a fast, 16-channel, 12-bit, single-supply,
analog-to-digital converter. The ADC is capable of throughput
rates of up to 1 MSPS when provided with a 20 MHz clock.
The ADC section provides the user with an on-chip track-
and-hold, analog-to-digital converter. The ADC section has
16 single-ended input channels with a channel sequencer,
allowing the user to select a sequence of channels through
which the ADC can cycle with each consecutive ASYNC falling
edge. The serial clock input accesses data from the ADC, controls
the transfer of data written to the ADC, and provides the clock
source for the successive approximation ADC converter. The
analog input range for the ADC is 0 V to VREFA or 0 V to 2 ×
VREFA depending on the status of Bit 1 in the control register.
The ADC provides flexible power management options to
allow the user to achieve the best power performance for a
given throughput rate. These options are selected by program-
ming the power management bits in the ADC control register.
ADC CONVERTER OPERATION
The ADC is a 12-bit successive approximation analog-to-digital
converter based around a capacitive DAC. The ADC can convert
analog input signals in the range 0 V to VREFA or 0 V to 2 × VREFA.
The ADC comprises control logic, SAR, and a capacitive DAC,
which are used to add and subtract fixed amounts of charge
from the sampling capacitor to bring the comparator back
into a balanced condition.
Figure 54 shows the ADC during
its acquisition phase. SW2 is closed and SW1 is in Position
A. The comparator is held in a balanced condition and the
sampling capacitor acquires the signal on the selected VIN
channel.
VIN0
VIN15
ADCGND
A
B
SW1
SW2
4k
COMPARATOR
CAPACITIVE
DAC
CONTROL
LOGIC
07691-
054
Figure 54. ADC Acquisition Phase
and SW1 moves to Position B, causing the comparator to become
unbalanced. The control logic and the capacitive DAC are used
to add and subtract fixed amounts of charge from the sampling
capacitor to bring the comparator back into a balanced condi-
tion. When the comparator is rebalanced, the conversion is
complete. The control logic generates the ADC output code.
VIN0
VIN15
ADCGND
A
B
SW1
SW2
4k
COMPARATOR
CAPACITIVE
DAC
CONTROL
LOGIC
07691-
055
Figure 55. ADC Conversion Phase
Analog Input
Figure 56 shows an equivalent circuit of the analog input structure
of the ADC. The two diodes, D1 and D2, provide ESD protection
for the analog inputs. Care must be taken to ensure that the analog
input signal never exceed the supply rails by more than 200 mV.
This causes these diodes to become forward biased and start
conducting current into the substrate. 10 mA is the maximum
current these diodes can conduct without causing irreversible
4 pF and can primarily be attributed to pin capacitance. Resistor
R1 is a lumped component made up of the on resistance of a
switch (track-and-hold switch) and also includes the on resis-
tance of the input multiplexer.
C1
4pF
VINx
CONVERSION PHASE—SWITCH OPEN
TRACK PHASE—SWITCH CLOSED
ADCVDD
R1
C2
30pF
D2
D1
07691-
056
Figure 56. Equivalent Analog Input Circuit
The total resistance is typically about 400 . Capacitor C2 is
the ADC sampling capacitor and typically has a capacitance of
30 pF. For ac applications, removing high frequency components
from the analog input signal is recommended by use of an RC
low-pass filter on the relevant analog input pin. In applications
where harmonic distortion and signal-to-noise ratio are critical,
drive the analog input from a low impedance source. Large
source impedances significantly affect the ac performance of
the ADC. This may necessitate the use of an input buffer
amplifier. The choice of the op amp is a function of the
particular application.
When no amplifier is used to drive the analog input, limit the
source impedance to low values. The maximum source impedance
depends on the amount of total harmonic distortion (THD) that
can be tolerated. The THD increases as the source impedance