ADT7518
 
Rev. A | Page 23 of 40 
The second method is applicable when the part is in round 
robin measurement mode. The part measures both the internal 
and external temperature sensors as it cycles through all pos-
sible measurement channels. The two temperature channels are 
measured each time the part runs a round robin sequence. In 
round robin mode, the part continuously measures all channels. 
Temperature measurement is also initiated after every read or 
write to the part when the part is in either single-channel 
measurement mode or round robin measurement mode.  
Once serial communication has started, any conversion in 
progress is stopped and the ADC is reset. Conversion starts 
again immediately after the serial communication has finished. 
The temperature measurement proceeds normally as described 
in the preceding section. 
V
DD 
 Monitoring 
The ADT7518 also has the ability to monitor its own power 
supply. The part measures the voltage on its V
DD
 pin to a 
resolution of 10 bits. The resulting value is stored in two 8-bit 
registers; the two LSBs are stored in register address 03h and the 
eight MSBs are stored in Register Address 06h. This allows the 
option of doing just a 1-byte read if 10-bit resolution is not 
important. The measured result is compared with the V
HIGH
 and 
V
LOW
 limits. If the V
DD
 interrupt is not masked, any out-of-limit 
comparison generates a flag in the Interrupt Status 2 register 
and one or more out-of-limit results will cause the INT/
INT
 
output to pull either high or low, depending on the output 
polarity setting. 
Measuring the voltage on the V
DD
 pin is regarded as monitoring 
a channel along with the internal, external, and AIN channels. 
The user can select the V
DD
 channel for single-channel 
measurement by setting Bit C4 = 1 and setting Bits C0: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 8 shows the data 
format. As the maximum V
DD
 voltage measurable is 7 V, internal 
scaling is performed on the V
DD
 voltage to match the 2.25 V 
internal reference value. Below is an example of how the 
transfer function works. 
VDD = 5 V 
   ADC Reference = 2.25 V 
   1 LSB = ADC Reference/2
10
 
         = 2.25/1024 
         = 2.226 mV 
   Scale Factor = Full-Scale V
CC
/ADC Reference 
         = 7/2.25 
         = 3.07 
   Conversion Result = VDD/(Scale Factor ?LSB size) 
         = 5/(3.07 ?2.226 mV) 
         = 2 DCh 
 
Table 8. VDD Data Format (VREF = 2.25 V)  
 
Digital Output 
V
DD 
Value (V) 
Binary  
Hex  
2.7  
01 1000 1011  
18B 
3  
01 1011 0111  
1B7  
3.5  
10 0000 0000  
200  
4  
10 0100 1001  
249  
4.5  
10 1001 0010  
292  
5  
10 1101 1100  
2DC  
5.5  
11 0010 0101  
325  
6  
11 0110 1110  
36E  
6.5  
11 1011 0111  
3B7  
7  
11 1111 1111  
3FF  
On-Chip Reference 
The ADT7518 has an on-chip 1.2 V band gap reference, which 
is gained up by a switched capacitor amplifier to give an output 
of 2.25 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. The ADC is used 
for measuring VDD, internal temperature sensor, external temp-
erature sensor, and AIN inputs. The internal reference is always 
used when measuring V
DD
, and the internal and external temp-
erature sensors. The external reference is the default power-up 
reference for the DACs. 
Round Robin Measurement 
On power-up, the ADT7518 goes into round robin mode but 
monitoring is disabled. Setting Bit C0 of the Configuration 
Register 1 to 1 enables conversions. It sequences through all the 
available channels, taking a measurement from each in the 
following order: VDD, internal temperature sensor, external 
temperature sensor/(AIN1 and AIN2), AIN3, and AIN4. Pin 7 
and Pin 8 can be configured to be either external temperature 
sensor pins or standalone analog input pins. Once conversion is 
completed on the AIN4 channel, the device loops around for 
another measurement cycle. This method of taking a measure-
ment on all the channels in one cycle is called round robin. 
Setting Bit C4 of Control Configuration 2 (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, e.g., the internal temperature sensor, is measured in 
each conversion cycle.  
The time taken to monitor all channels will normally not be of 
interest, since the most recently measured value can be read at 
any time. For applications where the round robin time is impor-
tant, typical times at 25癈 are given in the specifications. 
Single-Channel Measurement 
Setting C4 of the Control Configuration 2 register enables the 
single-channel mode and allows the ADT7518 to focus on one 
channel only. A channel is selected by writing to Bits C0:C2 in 
the Control Configuration 2 register. For example, to select the 
V
DD
 channel for monitoring, write to the Control Configuration