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AD7817ARZ Datasheet(PDF) 10 Page - Analog Devices |
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AD7817ARZ Datasheet(HTML) 10 Page - Analog Devices |
10 / 20 page REV. C –10– 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 1LSB 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. 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. 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 Overtemperature 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. 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 00 0Temperature Sensor All 00 1Channel 1 AD7817/AD7818 01 0Channel 2 AD7817 01 1Channel 3 AD7817 10 0Channel 4 AD7817 11 1Internal Ref (1.23 V) All Overtemperature 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 Overtemperature Register—see Figure 4. At the end of a tem- perature conversion a digital comparison is carried out between the 8 MSBs of the temperature conversion result (10 bits) and the contents of the Overtemperature Register (8 bits). If the result of the temperature conversion is greater that the contents of the Overtemperature Register (OTR), then the Overtemperature 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. DB0 DB1 DB2 DB3 DB4 DB5 DB6 DB7 MSB LSB CONTROL BYTE DB0 DB1 DB2 ADDRESS REGISTER OVERTEMPERATURE REGISTER (OTR) 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 OVERTEMPERATURE REGISTER IF DB7 TO DB3 ARE LOGIC 0 THEN DB2 TO DB0 ARE WRITTEN TO THE ADDRESS REGISTER DB0 DB1 DB2 DB3 DB4 DB5 DB6 DB7 Figure 4. Address and Overtemperature Register Selection |
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