ADT7475 datasheet(13/68 Pages) AD

13 / 68 page

ADT7475

Rev. A | Page 13 of 68

INPUT CIRCUITRY

The internal structure for the VCCP analog input is shown in

Figure 20. The input circuit consists of an input protection

diode, an attenuator, and a capacitor to form a first-order, low-

pass filter that gives the input immunity to high frequency

noise.

VCCP

17.5kΩ

52.5kΩ

35pF

Figure 20. Structure of Analog Inputs

VOLTAGE MEASUREMENT REGISTERS

VCCP LIMIT REGISTERS

Associated with the VCCP measurement channel is a high and

low limit register. Exceeding the programmed high or low limit

causes the appropriate status bit to be set. Exceeding either limit

can also generate SMBALERT interrupts.

Table 5 shows the input ranges of the analog inputs and output

codes of the 10-bit ADC.

When the ADC is running, it samples and converts a voltage

input in 711 µs and averages 16 conversions to reduce noise; a

measurement takes nominally 11.38 ms.

EXTENDED RESOLUTION REGISTERS

Voltage measurements can be made with higher accuracy using

the extended resolution registers (0x76 and 0x77). Whenever

the extended resolution registers are read, the corresponding

data in the voltage measurement registers is locked until their

data is read. That is, if extended resolution is required, then the

extended resolution register must be read first, immediately

followed by the appropriate voltage measurement register.

ADDITIONAL ADC FUNCTIONS FOR VOLTAGE

MEASUREMENTS

A number of other functions are available on the ADT7475 to

offer the system designer increased flexibility.

Turn-Off Averaging

For each voltage measurement read from a value register,

16 readings have been made internally, and the results averaged,

before being placed into the value register. For instances where

faster conversions are needed, setting Bit 4 of Configuration

reading 16 times faster (711 µs), but the reading may be noisier.

Bypass Voltage Input Attenuator

Setting Bit 5 of Configuration Register 2 (0x73) removes the

attenuation circuitry from the VCCP input. This allows the user

to directly connect external sensors, or to rescale the analog

voltage measurement inputs for other applications. The input

range of the ADC without the attenuators is 0 V to 2.25 V.

Single Channel ADC Conversion

Setting Bit 6 of Configuration Register 2 (0x73) places the

ADT7475 into single channel ADC conversion mode. In this

mode, the ADT7475 can be made to read a single voltage chan-

nel only. If the internal ADT7475 clock is used, the selected

input is read every 711 µs. The appropriate ADC channel is

selected by writing to Bits [7:5] of the TACH1 minimum high

byte register (0x55).

Table 4. Single Channel ADC Conversion

Channel Selected

001

VCCP

010

VCC

101

Remote 1 Temperature

110

Local Temperature

111

Remote 2 Temperature

Configuration Register 2 (0x73)

Bit 4 = 1; averaging off.

Bit 5 = 1; bypass input attenuators.

Bit 6 = 1; single channel convert mode.

TACH1 Minimum High Byte (0x55)

Bits [7:5] selects ADC channel for single channel convert mode.

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