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AD7818ARM
(Rev.:2000)

Single- and 4-Channel, 9 μ
s, 10-Bit ADCs with On-Chip Temperature Sensor

Analog Devices 

AD7816/AD7817/AD7818

The AD7816, AD7817 and AD7818 are tested using the CCIF

standard where two input frequencies near the top end of the

input bandwidth are used. In this case, the second and third

order terms are of different significance. The second order terms

are usually distanced in frequency from the original sine waves

while the third order terms are usually at a frequency close to

the input frequencies. As a result, the second and third order

terms are specified separately. The calculation of the intermodu-

lation distortion is as per the THD specification where it is the

ratio of the rms sum of the individual distortion products to the

rms amplitude of the fundamental expressed in dBs.

Channel-to-Channel Isolation

Channel-to-channel isolation is a measure of the level of

crosstalk between channels. It is measured by applying a full-

scale 20␣ kHz sine wave signal to one input channel and deter-

mining how much that signal is attenuated in each of the other

channels. The figure given is the worst case across all four

channels.

Relative Accuracy

Relative accuracy or endpoint nonlinearity is the maximum

deviation from a straight line passing through the endpoints of

the ADC transfer function.

Differential Nonlinearity

This is the difference between the measured and the ideal

1␣ LSB change between any two adjacent codes in the ADC.

Offset Error

This is the deviation of the first code transition (0000 . . . 000)

to (0000 . . . 001) from the ideal, i.e., AGND + 1 LSB.

Address Register

If the five MSBs of the control byte are logic zero, the three

LSBs of the control byte are transferred to the Address Regis-

ter—see Figure 4. The Address Register is a 3-bit-wide register

used to select the analog input channel on which to carry out a

conversion. It is also used to select the temperature sensor,

which has the address 000. Table I shows the selection. The

Internal Reference selection connects the input of the ADC to a

band gap reference. When this selection is made and a conver-

sion is initiated, the ADC output should be approximately mid-

scale. After power-up the default channel selection is DB2 = DB1

= DB0 = 0 (Temperature Sensor).

Table I. Channel Selection

DB2 DB1 DB0 Channel Selection Device

0

0

0

Temperature Sensor All

0

0

1

Channel 1

AD7817/18

0

1

0

Channel 2

AD7817

0

1

1

Channel 3

AD7817

1

0

0

Channel 4

AD7817

1

1

1

Internal Ref (1.23 V) All

Over-Temperature Register

If any of the five MSBs of the control byte are logic one then the

entire eight bits of the control byte are transferred to the Over-

Temperature Register—see Figure 4. At the end of a tempera-

ture conversion a digital comparison is carried out between the

DB2 DB1 DB0 ADDRESS REGISTER

Offset Error Match

This is the difference in Offset Error between any two channels.

Gain Error

This is the deviation of the last code transition (1111 . . . 110) to

(1111 . . . 111) from the ideal, i.e., VREF – 1 LSB, after the

offset error has been adjusted out.

IF DB7 TO DB3 ARE LOGIC '0'

THEN DB2 TO DB0 ARE WRITTEN

TO THE ADDRESS REGISTER

MSB

DB7 DB6 DB5 DB4 DB3 DB2

LSB

DB1 DB0 CONTROL BYTE

Gain Error Match

This is the difference in Gain Error between any two channels.

Track/Hold Acquisition Time

Track/hold acquisition time is the time required for the output

of the track/hold amplifier to reach its final value, within

± 1/2 LSB, after the end of conversion (the point at which the

track/hold returns to track mode). It also applies to situations

where a change in the selected input channel takes place or

where there is a step input change on the input voltage applied

to the selected VIN input of the AD7817 or AD7818. It means

that the user must wait for the duration of the track/hold acqui-

sition time after the end of conversion or after a channel change/

step input change to VIN before starting another conversion, to

ensure that the part operates to specification.

CONTROL BYTE

The AD7816, AD7817 and AD7818 contain two on-chip regis-

ters, the Address Register and the Over-Temperature Register.

These registers can be accessed by carrying out an 8-bit serial

write operation to the devices. The 8-bit word or control byte

written to the AD7816, AD7817 and AD7818 is transferred to

one of the two on-chip registers as follows.

IF ANY BIT DB7 TO DB3 IS SET TO

A LOGIC '1' THEN THE FULL 8 BITS

OF THE CONTROL WORD ARE WRITTEN

TO THE OVER-TEMPERATURE REGISTER

DB7

DB6

DB5

DB4

DB3 DB2

DB1

DB0

OVER-TEMPERATURE

REGISTER (OTR)

Figure 4. Address and Over-Temperature Register Selection

8 MSBs of the temperature conversion result (10 bits) and the

contents of the Over-Temperature Register (8 bits). If the result

of the temperature conversion is greater that the contents of the

Over-Temperature Register (OTR), then the Over-Temperature

Indicator (OTI) goes logic low. The resolution of the OTR is

1°C. The lowest temperature that can be written to the OTR is

–95°C and the highest is +152°C—see Figure 5. However, the

usable temperature range of the temperature sensor is –55°C to

+125°C. Figure 5 shows the OTR and how to set TALARM (the

temperature at which the OTI goes low).

OTR (Dec) = TALARM (°C) + 103°C

For example, to set TALARM to 50°C, OTR = 50 + 103 = 153

Dec or 10011001 Bin. If the result of a temperature conversion

exceeds 50°C then OTI will go logic low. The OTI logic output

is reset high at the end of a serial read operation or if a new

temperature measurement is lower than TALARM. The default

power on TALARM is 50°C.

REV. A

–9–

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