AN607

Rev. 1.7

37

The command code, 0xC5, is used for programming; so, for example, to program a Si7013 at slave address 0x40

with the values above starting with 0x4C to memory location 0x82, one would write:

<Start Condition> 0x40 W ACK 0xC5 ACK 0x82 ACK 0x2E ACK <Stop Condition>

……

<Start Condition> 0x40 W ACK 0xC5 ACK 0xB7 ACK 0xD6 ACK <Stop Condition>

the correction mode of the voltage-sensing circuit.

It is not possible to change the values once written. However, to verify the values were written properly, use

command 0x84. In the above example, to verify that 0x2E was written to location 0x82, use:

<Start Condition> 0x40 W ACK 0x84 ACK 0x82 ACK <Start Condition> 0x40R ACK 0x2E NACK <Stop Condition>

where 0x2E is the expected return value of the read transaction.

How to Choose Linearization Points

The table must be arranged in order of decreasing values for the input. The slope can be positive or negative, and

it is assumed that only positive output values are desired.

Generally, the input/output pairs should be chosen around the area where greatest accuracy is desired. If the

transfer function is highly nonlinear and the more closely spaced the points, the less error there will be from

interpolation. However, if accuracy is desired over a wide range, the input/output pairs should be spaced evenly

over the range of interest. The spreadsheet allows estimation of errors from the extrapolation process.

Other Register Settings

The voltage measurement configuration is determined by user register 2 of the Si7013. For thermistor

measurement, it is assumed that the A/D input buffers will be used and that the A/D reference is VDD. The buffers

are needed since the thermistor impedance is fairly large. VDD is used as a reference because the thermistor is

biased relative to VDD. Thus, for a “hold master” measurement (SCL is held low during the measurement phase),

Register 2 would be set to the following:

Noise and AC Pick Up

The A/D of the Si7013 is a delta sigma type converter, and the input is not sampled. Thus, it is assumed that the

input voltage is constant over the measurement period. Generally, some amount of analog filtering prior to the A/D

input is desirable. In the standard application circuit, this is accomplished with 0.1 µF capacitors. These capacitors

will form a filter at about 30 Hz, which is adequate for high-frequency noise pick up (e.g. am radio signals) but not

for 60 Hz. If 60 Hz filtering is desired, these capacitors can be increased to >1 µF, or the result could be digitally

filtered (average of several measurements). If the sampling can be synchronized to 120 Hz (use the faster

conversion time for this), then an average of just two samples would reject 60 Hz.

In the standard biasing circuit, the bias can be turned off between measurements to save power (change bit zero of

user register 2). If this is done, allow adequate settling time between enabling the bias and making the

measurement (use approximately 100 msec for the 0.1 µF filter, which has a time constant of 30 msec).

D7

D6

D5

D4

D3

D2

D1

D0

00

1

0

0

1

1

0/1

Not Used

Hold

Master

Enable

Linearization

7 msec

Conversion Time

Not Used

Enable

Buffer

VDD is

Reference

Switch High or Low to

Bias the Thermistor