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ADT7411 Datasheet(PDF) 17 Page - Analog Devices

Part No. ADT7411
Description  SPI-/IC-Compatible, 10-Bit Digital Temperature Sensor and 8-Channel ADC
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

ADT7411 Datasheet(HTML) 17 Page - Analog Devices

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ADT7411
Rev. B | Page 17 of 36
VDD Monitoring
The ADT7411 also has the capability of monitoring its own
power supply. The part measures the voltage on its VDD pin to a
resolution of 10 bits. The resulting value is stored in two 8-bit
registers, with the 2 LSBs stored in register Address 03h and the
8 MSBs stored in register Address 06h. This allows the user to
have the option of just doing a 1-byte read if 10-bit resolution is
not important. The measured result is compared with the VHIGH
and VLOW limits. If the VDD interrupt is not masked out then any
out-of-limit comparison generates a flag in the Interrupt Status 2
register, and one or more out-of-limit results causes the
INT/INT output to pull either high or low, depending on the
output polarity setting.
Measuring the voltage on the VDD pin is regarded as monitoring
a channel along with the internal, external, and AIN channels.
The user can select the VDD channel for single-channel
measurement by setting Bit C4 = 1 and by setting Bit C0 to
Bit C2 to all 0s in the Control Configuration 2 register.
When measuring the VDD value, the reference for the ADC is
sourced from the internal reference. Table 5 shows the data
format. As the maximum VDD voltage measurable is 7 V,
internal scaling is performed on the VDD voltage to match the
2.25 V internal reference value. The following is an example of
how the transfer function works.
ADC Reference = 2.25 V
1 LSB = ADC Reference/210 = 2.25/1024 = 2.197 mV
Scale Factor = Full Scale VCC/ADC Reference = 7/2.25 = 3.11
Conversion Result = VDD/(Scale Factor × LSB Size)
= 5/(3.11 × 2.197 mV)
= 2DBh
Table 5. VDD Data Format, VREF = 2.25 V
Digital Output
VDD Value (V)
Binary
Hex
2.5
01 0110 1110
16E
2.7
01 1000 1011
18B
3.0
01 1011 0111
1B7
3.5
10 0000 0000
200
4.0
10 0100 1001
249
4.5
10 1001 0010
292
5.0
10 1101 1011
2DB
5.5
11 0010 0100
324
6.0
11 0110 1101
36D
6.5
11 1011 0110
3B6
7.0
11 1111 1111
3FF
On-Chip Reference
The ADT7411 has an on-chip 1.125 V band gap reference that
is gained up by a switched capacitor amplifier to give an output
of 2.25 V. The amplifier is powered up for the duration of the
device monitoring phase and is powered down once monitoring
is disabled. This saves on current consumption. The internal
reference is used as the reference for the ADC.
Round Robin Measurement
Upon power-up, the ADT7411 goes into round robin mode but
monitoring is disabled. Setting Bit C0 of the Configuration 1
register to 1 enables conversions. It sequences through all
available channels, taking a measurement from each in the
following order: VDD, internal temperature sensor, external
temperature sensor/(AIN1 and AIN2), AIN3, AIN4, AIN5,
AIN6, AIN7, and AIN8. Pin 7 and Pin 8 can be configured as
either external temperature sensor pins or standalone analog
input pins. Once conversion is completed on the AIN8 channel,
the device loops around for another measurement cycle.
This method of taking a measurement on all the channels in
one cycle is called round robin. Setting Bit 4 of the Control
Configuration 2 register (Address 19h) disables the round robin
mode and in turn sets up the single-channel mode. The single-
channel mode is where only one channel, for example, the
internal temperature sensor, is measured in each conversion cycle.
The time taken to monitor all channels will normally not be of
interest, as the most recently measured value can be read at any
time. For applications where the round robin time is important,
typical times at 25°C are given in Table 1.
Single-Channel Measurement
Setting Bit C4 of the Control Configuration 2 register enables
the single-channel mode and allows the ADT7411 to focus on
one channel only. A channel is selected by writing to Bit C0 to
Bit C3 in the Control Configuration 2 register. For example,
to select the VDD channel for monitoring, write to the Control
Configuration 2 register and set C4 to 1 (if not done so already),
then write all 0s to Bit C0 to Bit C3. All subsequent conversions
are done on the VDD channel only. To change the channel selection
to the internal temperature channel, write to the Control
Configuration 2 register and set C0 = 1. When measuring in
single-channel mode, conversions on the channel selected occur
directly after each other. Any communication to the ADT7411
stops the conversions, but they are restarted once the read or
write operation is completed.


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