AD7416-7418 Analog Devices, AD7416-7418 Datasheet - Page 8

• Manufacturer Part Number: AD7416-7418
• Description: 10-Bit Digital Temperature Sensor (AD7416) and Single/Four-Channel ADC (AD7417/AD7418)
• Manufacturer: Analog Devices

AD7416/AD7417/AD7418
CIRCUIT INFORMATION
The AD7417 and AD7418 are single- and four-channel, 15 s
conversion time, 10-bit A/D converters with on-chip tempera-
ture 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 A/D converter
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 –55 C to +125 C. The parts require a +2.5 V refer-
ence which can be provided from the part’s own internal refer-
ence 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 no external clock is required except when reading
from and writing to the serial port. The on-chip track/hold goes
from track 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/hold goes into hold approximately 3 s
after the read or write operation is complete and a conversion is
then initiated. The result of the conversion is available either
15 s or 30 s later, depending on whether an analog input
channel or the temperature sensor is selected. The track/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 s to complete.
Temperature measurement is explained in the Temperature
Measurement section of this data sheet.
The on-chip reference is not available to the user, but REF
can be overdriven by an external reference source (+2.5 V only).
The effect of reference tolerances on temperature measurements
is discussed in the Reference section of the data sheet.
All unused analog inputs should be tied to a voltage within the
nominal analog input range to avoid noise pickup. For mini-
mum power consumption, the unused analog inputs should be
tied to GND.
TYPICAL CONNECTION DIAGRAM
Figure 2 shows a typical connection diagram for the AD7417.
Using the A0, A1 and A2 pins allows the user to select from up
to eight AD7417s on the same serial bus, if desired. An external
2.5 V reference can be connected at the REF
nal reference is used, a 10 F capacitor should be connected
between REF
I
sumption is of concern, the automatic power-down at the end of
a conversion should be used to improve power performance. See
Operating Modes section of the data sheet.
2
C-compatible interface. For applications where power con-
IN
and GND. SDA and SCL form the two-wire
IN
pin. If an exter-
IN
–8–
ANALOG INPUTS
Figure 3 shows an equivalent circuit of the analog input struc-
ture 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. This will cause these diodes
to become 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. The
capacitor C2 in Figure 3 is typically about 4 pF and can prima-
rily be attributed to pin capacitance. The resistor R1 is a
lumped component made up of the on resistance of a multi-
plexer and a switch. This resistor is typically about 1
capacitor C1 is the ADC sampling capacitor and has a capaci-
tance of 3 pF.
ON-CHIP REFERENCE
The AD7416/AD7417/AD7418 has an on-chip 1.2 V band-gap
reference which is gained up 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
4) to open and the reference amplifier to power up during a
conversion. Therefore the on-chip reference is not available
externally. An external 2.5 V reference can be connected to the
REF
reference circuitry.
REFERENCE
EXTERNAL
A
IN
OPTIONAL
IN
pin. This has the effect of shutting down the on-chip
+2.7V TO
IN
SUPPLY
4pF
Figure 3. Equivalent Analog Input Circuit
C2
0V TO +2.5V
+5.5V
pin to analog ground. This causes SW1—(see Figure
Figure 2. Typical Connection Diagram
REF-192
INPUT
AD780/
V
DD
10 F
D1
D2
AIN2
AIN3
AIN4
GND
AIN1
0.1 F
CONVERT PHASE – SWITCH OPEN
TRACK PHASE – SWITCH CLOSED
AD7417
REF
V
1k
DD
R1
IN
CONVST
10 F FOR
EXTERNAL
REFERENCE
SCL
SDA
OTI
INTERFACE
TWO-WIRE
A0
A1
A2
SERIAL
3pF
C1
V
BALANCE
k
. The
C/ P
REV. B