AD7816-7818 Analog Devices, AD7816-7818 Datasheet - Page 9

• Manufacturer Part Number: AD7816-7818
• Description: Single- and 4-Channel/ 9 us/ 10-Bit ADCs with On-Chip Temperature Sensor
• Manufacturer: Analog Devices

CONTROL BYTE
The AD7816, AD7817 and AD7818 contain two on-chip regis-
ters, the Address Register and the Over-Temperature Register.
These registers can be accessed by carrying out an 8-bit serial
write operation to the devices. The 8-bit word or control byte
written to the AD7816, AD7817 and AD7818 is transferred to
one of the two on-chip registers as follows.
REV. A
The AD7816, AD7817 and AD7818 are tested using the CCIF
standard where two input frequencies near the top end of the
input bandwidth are used. In this case, the second and third
order terms are of different significance. The second order terms
are usually distanced in frequency from the original sine waves
while the third order terms are usually at a frequency close to
the input frequencies. As a result, the second and third order
terms are specified separately. The calculation of the intermodu-
lation distortion is as per the THD specification where it is the
ratio of the rms sum of the individual distortion products to the
rms amplitude of the fundamental expressed in dBs.
Channel-to-Channel Isolation
Channel-to-channel isolation is a measure of the level of
crosstalk between channels. It is measured by applying a full-
scale 20 kHz sine wave signal to one input channel and deter-
mining how much that signal is attenuated in each of the other
channels. The figure given is the worst case across all four
channels.
Relative Accuracy
Relative accuracy or endpoint nonlinearity is the maximum
deviation from a straight line passing through the endpoints of
the ADC transfer function.
Differential Nonlinearity
This is the difference between the measured and the ideal
1 LSB change between any two adjacent codes in the ADC.
Offset Error
This is the deviation of the first code transition (0000 . . . 000)
to (0000 . . . 001) from the ideal, i.e., AGND + 1 LSB.
Offset Error Match
This is the difference in Offset Error between any two channels.
Gain Error
This is the deviation of the last code transition (1111 . . . 110) to
(1111 . . . 111) from the ideal, i.e., VREF – 1 LSB, after the
offset error has been adjusted out.
Gain Error Match
This is the difference in Gain Error between any two channels.
Track/Hold Acquisition Time
Track/hold acquisition time is the time required for the output
of the track/hold amplifier to reach its final value, within
track/hold returns to track mode). It also applies to situations
where a change in the selected input channel takes place or
where there is a step input change on the input voltage applied
to the selected V
that the user must wait for the duration of the track/hold acqui-
sition time after the end of conversion or after a channel change/
step input change to V
ensure that the part operates to specification.
1/2 LSB, after the end of conversion (the point at which the
IN
input of the AD7817 or AD7818. It means
IN
before starting another conversion, to
–9–
Address Register
If the five MSBs of the control byte are logic zero, the three
LSBs of the control byte are transferred to the Address Regis-
ter—see Figure 4. The Address Register is a 3-bit-wide register
used to select the analog input channel on which to carry out a
conversion. It is also used to select the temperature sensor,
which has the address 000. Table I shows the selection. The
Internal Reference selection connects the input of the ADC to a
band gap reference. When this selection is made and a conver-
sion is initiated, the ADC output should be approximately mid-
scale. After power-up the default channel selection is DB2 = DB1
= DB0 = 0 (Temperature Sensor).
DB2
0
0
0
0
1
1
Over-Temperature Register
If any of the five MSBs of the control byte are logic one then the
entire eight bits of the control byte are transferred to the Over-
Temperature Register—see Figure 4. At the end of a tempera-
ture conversion a digital comparison is carried out between the
Figure 4. Address and Over-Temperature Register Selection
8 MSBs of the temperature conversion result (10 bits) and the
contents of the Over-Temperature Register (8 bits). If the result
of the temperature conversion is greater that the contents of the
Over-Temperature Register (OTR), then the Over-Temperature
Indicator (OTI) goes logic low. The resolution of the OTR is
1 C. The lowest temperature that can be written to the OTR is
–95 C and the highest is +152 C—see Figure 5. However, the
usable temperature range of the temperature sensor is –55 C to
+125 C. Figure 5 shows the OTR and how to set T
temperature at which the OTI goes low).
For example, to set T
Dec or 10011001 Bin. If the result of a temperature conversion
exceeds 50 C then OTI will go logic low. The OTI logic output
is reset high at the end of a serial read operation or if a new
temperature measurement is lower than T
power on T
MSB
DB7
DB7
DB6
TO THE OVER-TEMPERATURE REGISTER
DB6
OF THE CONTROL WORD ARE WRITTEN
DB1
0
0
1
1
0
1
A LOGIC '1' THEN THE FULL 8 BITS
IF ANY BIT DB7 TO DB3 IS SET TO
ALARM
DB5
DB5
OTR (Dec) = T
DB0
Table I. Channel Selection
is 50 C.
0
1
0
1
0
1
DB4
DB4
AD7816/AD7817/AD7818
ALARM
DB3
DB3
THEN DB2 TO DB0 ARE WRITTEN
IF DB7 TO DB3 ARE LOGIC '0'
TO THE ADDRESS REGISTER
Channel Selection
Temperature Sensor
Channel 1
Channel 2
Channel 3
Channel 4
Internal Ref (1.23 V)
to 50 C, OTR = 50 + 103 = 153
DB2
DB2
DB2
ALARM
DB1
DB1
DB1
( C) + 103 C
DB0
DB0
LSB
DB0
ALARM
ADDRESS REGISTER
CONTROL BYTE
OVER-TEMPERATURE
REGISTER (OTR)
. The default
ALARM
Device
All
AD7817/18
AD7817
AD7817
AD7817
All
(the