AD7745/46/47 addressing for multiple slave devices on I2C bus

Q I want to connect several Capacitance to Digital Converters (AD7745, AD7746, AD7747) using the I2C bus. The question is how can I get the slave address of each device or how can set the slave address to each one? A The I2C address of the AD7745/6/7 cannot be changed. In order to address multiple devices an I2C multiplexer is required to switch between the Capacitance to Digital Converters.

Long Term Drift Bahavior

Q What is the AD7745 long term drift behavior? A We guarantee accuracy within a 4fF band. In fact each part is factory tested to ensure that it is within a 2fF band; the extra 2fF then takes account of any aging effects. We have high confidence because of our own elevated temperature tests that were carried out when we characterised our silicon.

The absolute maximum ratings for AD7745 state the following;

Q The absolute maximum ratings for AD7745 state the following; I expect this device to be able to tolerate the Pb free solder temperature of 260C , Is AD7745 able to withstand up to 260C during Pb free soldering process? A Our parts are qualified to the reflow profile found in JEDEC specification J-STD-020C. The reflow profile the customer uses is dependent upon their board configuration but should not exceed the ramp rate or time at temp profile in J-STD-020C. AD7745 is qualified to withstand the higher peak Pb free reflow profile (up to 260C). The reflow profile in JEDEC specification J-STD-020C should be adhered to. It can be downloaded from JEDEC site at: http://www.jedec.org/standards-documents/docs/j-std-020d1 The data you refer to in your email in the absolute maximum ratings section of the AD7745 relates only to lead temperature for Vapor Phase (60 Sec) and Infrared (15 sec) and not to the Peak Reflow temperature. The peak reflow temperature for this device is listed on our materials declaration web page: http://quality.analog.com/ I have provided a screen shot below and you can see that peak reflow temperature is 260C. :

ad7745: Sensors with higher capacative range

Q They make humidiysensors with a capacitive output. Therefor, they have interests in CDC as AD7745. But the nominal capacitance of their sensors is 180pF with a max. change of +-50pF, which is higher than the AD7745 can handle. A 1) Connect a known fixed capacitor in series with the sensor. That would work by equation C = C1 x C2 / (C1 + C2). This method results in a non-linear relation between the sensor capacitance and the output data, but this can be corrected in the host uC software. Also, the AD7746 input range might not be fully used with implication of lower signal/noise ratio, but I believe the AD7746 itself has enough resolution to keep the system resolution on required level. My main concern would be how stable is the serial capacitor over temperature and temperature compensation for that. 2) Reduce the excitation voltage. The AD7746 basically measures charge and reducing the excitation voltage, proportionally higher capacitance can be measured.

Ad7745: Higher capacitive value

Q Is it possible to extend the range of measured capacitances above 4 pF by decreasing excitation signal e.g. to a value of VDD/4 ? A 1) Connect a known fixed capacitor in series with the sensor. That would work by equation C = C1 x C2 / (C1 + C2). This method results in a non-linear relation between the sensor capacitance and the output data, but this can be corrected in the host uC software. Also, the AD7746 input range might not be fully used with implication of lower signal/noise ratio, but I believe the AD7746 itself has enough resolution to keep the system resolution on required level. My main concern would be how stable is the serial capacitor over temperature and temperature compensation for that. 2) Reduce the excitation voltage. The AD7746 basically measures charge and reducing the excitation voltage, proportionally higher capacitance can be measured. See the attached pdf file for more details.

AD7745: there is no input protection circuit

Q Are there some protection circuit in front of the AD7745 to limit the input voltage and current while not affecting the capacitance measurement. A the AD7745 has no over voltage protection please refer to the absolute maximum ratings in the datasheet. you customer needs to ensure that a high voltage cannot be connected to the capacitive frontend

AD7745: Start a conversion in single conversion mode

Q How to start a conversion in single conversion mode? A How to start a conversion in single conversion mode: It is the operation of writing "010" to the mode bits MD2-MD0 in the configuration register which starts the conversion. There is no Start Convert pin or bit. Once the conversion result is available, the RDY\ pin (and the RDY bit in the status register) will go low and the MD2-MD0 bits will return to "000". For continuous conversion, RDY\ will go low when the conversion is complete and will return high when the result is read. If the result is not read before the following conversion result is available, the RDY\ pin goes high briefly before going low again to indicate the availability of the new conversion result. The CAPDAC full range (absolute value) is not factory calibrated and can vary up to ±20% with the manufacturing process. See Fig 17 in the datasheet.

AD7745: Higher input capacitance

Q The datasheet states that the max. common-mode cap for a AD7745 is 17pF. Is it possible to connect 50pF? Maybe with a voltage devider at EXCx? A 1) Connect a known fixed capacitor in series with the sensor. That would work by equation C = C1 x C2 / (C1 + C2). This method results in a non-linear relation between the sensor capacitance and the output data, but this can be corrected in the host uC software. Also, the AD7746 input range might not be fully used with implication of lower signal/noise ratio, but I believe the AD7746 itself has enough resolution to keep the system resolution on required level. My main concern would be how stable is the serial capacitor over temperature and temperature compensation for that. 2) Reduce the excitation voltage. The AD7746 basically measures charge and reducing the excitation voltage, proportionally higher capacitance can be measured.

AD7745: Capacitance Application

Q 1.:What is the difference between the AD7745 and the AD7747? Is it only the different resolution and accuracy and the input range? 2.:What is the difference between the models for the AD7746: ARUZ / REEL / REEL 7 ? 3.:If the resolution of the AD7746 is stated as down to 4aF, can I really measure a change in capacity of 4aF with it? 4.:Is it possible to measure capacities much larger than the specified maximum CAPDAC range of +13pF...+21pF for the AD7746? I read something of lowering the excitation voltage or using another capacity in series. Does that effect the resolution/accuracy much? 5.:If I want to use an evalution board, is it really that simple, that I only connect my capacity to the CDC and the CDC to the eval-board and that to the PC via USB? And then I get the capacity shown in the evaluation software? A 1 The major change of the Ad7747 is the Input range of 8pF ( as opposed to 4pF for the ad7745) The power consumption is also slightly less. 2 the ARUZ is a single device whereas the REEL is a standard reel of devices(2500 devices). The REEL7 is a smaller REEL width again. 3 Yes , 4pF is full scale, 4fFis Absolue Error - your mail is showing aF. 4 See application information in the datasheet for this information. But this is possible. 5 I can recommend the evaluation PCB, but this has its own sensor, where you slide a dielectric between plates. But you should be able to connect to the Eval PCB, once your Capacitor is in this region of operation.

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AD7745 Production
The AD7745/AD7746 are a high resolution, Î£-Î” capacitance-to-digital converter (CDC). The capacitance to be measured is connected directly to the device...
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