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AD7416/AD7417/AD7418

Rev. I | Page 18 of 24

CONVST Pin Mode

Mode 2

Conversions are initiated only by using the CONVST pin. In

this method of operation, CONVST is normally low.

For applications where temperature measurements are required

at a slower rate, for example, every second, power consumption

of the part can be reduced by writing to the part to go to a full

power-down between reads. The current consumption in full

power-down is typically 0.2 μA and full power-down is initiated

when D0 = 1 in the configuration register. When a measurement is

required, a write operation can be performed to power up the

part. The part then performs a conversion and is returned to

power-down. The temperature value can be read in full power-

down because the I2C bus is continuously active.

The rising edge of CONVST starts the power-up time. This

power-up time is 4 μs. If the CONVST high time is longer than

4 μs, a conversion is initiated on the falling edge of CONVST

and the track-and-hold also enters its hold mode at this time.

If the CONVST high time is less than 4 μs, an internal timer,

initiated by the rising edge of CONVST, holds off the track-

and-hold and the initiation of conversion until the timer times

out (4 μs after the rising edge of CONVST, which corresponds

with the power-up time). The CONVST input remains low at

the end of conversion, thus causing the part to enter its power-

down mode. In this method of operation, CONVST is normally

low with a high going pulse controlling the power-up, and the

conversion starts.

The power dissipation in this mode depends on the rate at which

reads take place. Taking the requirements for a temperature

measurement every 100 ms as an example, the optimum power

dissipation is achieved by placing the part in full power-down,

waking it up every 100 ms, letting it operate for 400 μs and

putting it into full power-down again. In this case, the average

power consumption is calculated as follows. The part spends

40 μs (or 0.04% of time) converting with 3 mW dissipation

and a 99.96 ms (99.96% of time) in full shutdown with 60 nW

dissipation.

The CONVST pin should not be pulsed when reading from or

writing to the port.

Figure 21 shows the recommended minimum times for the

CONVST pulse when the temperature channel is selected.

shows the minimum times an analog input channel is

selected.

Figure 22

Thus, the average power dissipation is

3 mW × 0.004 + 60 nW × 0.9996 = 1.2 μW

CONVST

100ns

40µs

The fastest throughput rate at which the AD7416/AD7417/

AD7418 can be operated is 2.5 kHz (that is, a read every 400 μs

conversion period). Because TOTI and THYST are 2-byte reads, the

read time with the I2C operating at 100 kbps would be 270 μs. If

temperature reads are called too often, reads will overlap with

conversions, aborting them continuously, which results in

invalid readings.

Figure 21. CONVST When Temperature Channel Selected

CONVST START MODE

CONVST

100ns

15µs

The AD7417/AD7418 have an extra mode, set by writing to the

MSB of the Config2 register.

Figure 22. CONVST When VIN Channel Selected

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