AD7416/AD7417/AD7418
Rev. I | Page 11 of 24
THEORY OF OPERATION
CIRCUIT INFORMATION
The AD7417 and AD7418 are single-channel and four-channel,
15 μs conversion time, 10-bit ADCs with on-chip temperature
sensor, reference, and serial interface logic functions on a single
chip. The AD7416 has no analog input channel and is intended
for temperature measurement only. The ADC section consists
of a conventional successive approximation converter based
around a capacitor DAC. The AD7416, AD7417, and AD7418
are capable of running on a 2.7 V to 5.5 V power supply, and the
AD7417 and AD7418 accept an analog input range of 0 V to
+VREF. The on-chip temperature sensor allows an accurate
measurement of the ambient device temperature to be made.
The working measurement range of the temperature sensor is
−40°C to +125°C. The parts require a 2.5 V reference that can
be provided from the part’s own internal reference or from an
external reference source.
CONVERTER DETAILS
Conversion is initiated on the AD7417/AD7418 by pulsing the
CONVST
input. The conversion clock for the part is internally
generated so that no external clock is required except when
reading from and writing to the serial port. The on-chip track-
and-hold goes from track mode to hold mode, and the conversion
sequence is started on the falling edge of the
CONVST
signal.
A conversion is also initiated in the automatic conversion mode
every time a read or write operation to the AD7416/AD7417/
AD7418 takes place. In this case, the internal clock oscillator
(which runs the automatic conversion sequence) is restarted
at the end of the read or write operation. The track-and-hold
goes into hold mode approximately 3 μs after the read or write
operation is complete, and a conversion is then initiated. The
result of the conversion is available either 15 μs or 30 μs later,
depending on whether an analog input channel or the tempera-
ture sensor is selected. The track-and-hold acquisition time of
the AD7417/AD7418 is 400 ns.
A temperature measurement is made by selecting the Channel 0
of the on-chip mux and carrying out a conversion on this
channel. A conversion on Channel 0 takes 30 μs to complete.
Temperature measurement is explained in the Temperature
Measurement section.
The on-chip reference is not available to the user, but REF
IN
can
be overdriven by an external reference source (2.5 V only).
All unused analog inputs should be tied to a voltage within the
nominal analog input range to avoid noise pickup. For
minimum power consumption, the unused analog inputs
should be tied to GND.
TYPICAL CONNECTION DIAGRAM
Figure 9 shows a typical connection diagram for the AD7417.
Using the A0, A1, and A2 pins allows the user to select from up
to eight AD7417 devices on the same serial bus, if desired. An
external 2.5 V reference can be connected at the REF
IN
pin. If an
external reference is used, a 10 μF capacitor should be connected
between REF
IN
and GND. SDA and SCL form the 2-wire I
2
C
compatible interface. For applications where power consump-
tion is of concern, the automatic power-down at the end of a
conversion should be used to improve power performance (see
the Operating Modes section.)
+ +
SUPPLY
2.7V TO 5.5V
2-WIRE
SERIAL
INTERFACE
0.1µF10µF
10µF FOR
EXTERNAL
REFERENCE
OPTIONAL
EXTERNAL
REFERENCE
AD780/
REF192
0V TO 2.5V
INPUT
SDA
SCL
GND
OTI
CONVST
V
DD
REF
IN
AD7417
A0
A1
A2
A
IN1
A
IN2
A
IN3
A
IN4
01126-008
MICROCONTROLLER/
MICROPROCESSOR
Figure 9. Typical AD7417 Connection Diagram
ANALOG INPUTS
Figure 10 shows an equivalent circuit of the analog input
structure of the AD7417 and AD7418. 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
exceeds the supply rails by more than 200 mV to prevent these
diodes from becoming forward-biased and start conducting
current into the substrate. The maximum current these diodes
can conduct without causing irreversible damage to the part is
20 mA. Capacitor C2 in Figure 10 is typically about 4 pF and
can primarily be attributed to pin capacitance. Resistor R1 is a
lumped component made up of the on resistance of a multiplexer
and a switch. This resistor is typically about 1 kΩ. Capacitor C1
is the ADC sampling capacitor and has a capacitance of 3 pF.
DD
V
BALANCE
IN
R1
1kΩ
CONVERT PHASE: SWITCH OPEN
TRACK PHASE: SWITCH CLOSED
D1
D2
C2
4pF
C1
3pF
01126-009
Figure 10. Equivalent Analog Input Circuit
ON-CHIP REFERENCE
The AD7417/AD7418 have an on-chip 1.2 V band gap reference
that is amplified by a switched capacitor amplifier to give an
output of 2.5 V. The amplifier is only powered up at the start of
the conversion phase and is powered down at the end of the
conversion. The on-chip reference is selected by connecting the
REF
IN
pin to analog ground, which causes SW1 (see Figure 11) to
open and the reference amplifier to power up during a conver-
sion. Therefore, the on-chip reference is not available externally.
