mbvalentin

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  1. As @D@n mentioned, I was able to make the code work. I am actually very thankful for all the help he provided me through the process. To be honest I am not entirely sure what I changed, exactly, but my guess is that the values were not read correctly from the registers of the AD5933. Here are some personal tips when using the pmodIA: 1) Be careful when picking the right combination of Feedback resistor (RFB), PGA Gain and Output Range voltage Something very interesting that I found is that the actual implementation in the pmodIA is the one recommended by Analog Devices on the CN0217 Circuit note (take a look here: EVAL-CN0217-EB1Z .pdf). The only difference between this circuit and the circuit implemented on the pmodia is that the feedback resistor (RFB) in the pmodia can be selected by the user using the SEL pin (this is achieved by using an ADG849 digital switch, see datasheet here: http://www.analog.com/media/en/technical-documentation/data-sheets/ADG849.pdf). If SEL goes to GND the feedback resistor RFB is 100k. If SEL goes to VDD then RFB is 20k. Now, this RFB is important because choosing the wrong value might cause your ADC to saturate and, therefore, to give you a strange response. In page 19 of the circuit note I added before they give the formula to get the gain of the circuit. After some calculations I'd actually say that that formula is not the gain, but rather THE VOLTAGE input that arrives to the ADC (but I could be wrong about that, of course). Nevertheless YOU SHOULD BE CAREFUL ENOUGH TO PICK THE RIGHT COMBINATION OF EXCITATION RANGE VOLTAGE (2000mVpp,200mVpp,etc), THE RIGHT FEEDBACK RESISTOR (USING SEL PIN) AND THE RIGHT PGA GAIN (X1,X5) for the UNKNOWN IMPEDANCE THAT YOU ARE WILLING TO MEASURE. 2) You must calibrate your system with a known impedance with a value close to the one you're willing to measure. know this might sound a little bit stupid but you have to know, at least, the range of impedance where you are going to work at. For instance, if you wanna measure an impedance that you guess might be around 180 kohm and 220kohm, then you must calibrate the system for those values. You can find an example of this at the circuit note I uploaded here, page 20, left column 3rd paragraph ("For example: assuming the following:..."). Do not calibrate your board with a 1MOhm resistor using a 300 kHz signal if you want to read impedances with a value close to a couple of hundred kohms and using a signal of 3 KHz... 3) Work in the narrowest bandwidth possible and, preferably, in mid-high frequencies. The pmodIA is based in the CN0217 which is a "high precision impedance converter". High precision usually comes to a price which I would say that, in this case, is the bandwidth. In the document I uploaded, page 22, they say that "... The AD5933 is specified to a typical system accuracy of 0.5% (assuming the AD5933 system is calibrated correctly for the impedance range under test) within the frequency range of 1 kHz up to 100 kHz...". However, all examples they give in the note uses a very narrow band (page 20, they give an example with a band between 30 kHz and 32 kHz, and they even say that you have to calibrate the board using a mid-point frequency of 31 kHz). This might have been one of the problems I had in my original code because I was using a 3 kHz signal with a 100k resistor to calibrate the system for a sweep between 3 kHz and 1 MHz. 4) Make sure the i2c reading/writing procedure is okay and all your readings are what they are suppose to be I used wiringPi library to communicate with the pmod in my code. Through the process @D@n and I discovered that wiringpi's i2cReadReg16 function was not working properly to read 2byte words from successive registers (like 0x94 and 0x95, for instance), so I had to stick to a double reading of 8 bits rather than try to read these measures at once. Anyway, here's my code (BACKUP_WORKING_REPEAT.tar.gz). I am still working on it but at least it might be helpful for anyone who wants to use Raspberry Pi to communicate with the pmodIA or directly to the AD5933. Hope it helps. Manu B. V.
  2. Hello to everyone. First of all let me say that I already posted this question, by mistake, in the 'New users introduction' forum. As I don't know how to move it to another section I decided to start this new thread. In case the administrators consider it necessary I will delete my last post. That said... I recently purchased an pmodIA board for a project we are conducting at work. The main goal is to obtain some resistivity vs frequency spectra for different objects as part of a object characterization problem involving Oil/Gas stuff. I already read the guide that digilent guys provide (http://store.digilentinc.com/pmod-ia-impedance-analyzer/) and I found some C libraries for the AD5933 (https://github.com/analogdevicesinc/no-OS/blob/master/Pmods/PmodIA/AD5933.c and https://github.com/analogdevicesinc/no-OS/blob/master/drivers/AD5933/AD5933.c). I am using a Raspberry Pi 3 Model B to control & communicate with the pmodIA. I based my code on those C libraries I mentioned. In order to use i2c protocol with the raspberry pi I use wiringPi library (http://wiringpi.com/reference/i2c-library/). In my code I have tested everything I've been able to, from all functionalities of the pmod: I get the temperature readings, set the frequency sweep params, make a freq. sweep measurement, etc. I have attached my code to this post (pmodia_test.tar.gz). All functionalities I tested seem to be okay, and everything seems to be working: I get the temperature, I can do a frequency sweep and if I connect the outputs of the pmod to an oscilloscope I can see the frequency sweeping (look at this video I recorded) Now, the thing is that even though everything seems to be WORKING, the result values I am getting back just make no sense AT ALL!!! For calibration purposes I am using a 12k resistor. In my code I first set the gain (X1) and range (2Vpp), I configure the sweep params and start it, then I calculate the gainfactor using that 12k resistor (which I later use on all my calculations), and then I ask the user to change the impedance and put the one that they desire to measure. On the main loop I simply do the frequency sweep, increasing the frequency at each step, calculate the impedance and put this value into an array (Z). I am also using gnuplot to display the results (the values obtained from the pmod are the exact same values that are being plotted using gnuplot, so this is not a display problem, as I checked this). This is when things get weird. These are the results I got for the following tests: The following curve is the IMPEDANCE response for the 12k resistor. At low frequencies there are quite a lot of spikes and high values. Notice that this curve is actually pretty close to the 'open circuit' ones from my first question in this post. The following curve is the MAGNITUDE for that same case. These spikes are caused by the lower value of Magnitude at low frequencies. Now, if I zoom to a range from 15kHz and 1MHz you can see that it actually displays something more reasonable (impedance going from 12k to almost 4k). I repeated the test but now using the 200mVpp output range instead of the 2000mVpp I was using, and the noise appeared back (see following curve). Am I maybe saturating the ADC or something? @D@n suggested me to check the part of my code that reads the data, because that noise or those readings might be related to the way I am handling with the signed/unsigned values stored in the ad5933 registers for the real/imaginary data. This is the part of my code that reads those registers: /****************************************************************************** * @brief Calculate impedance. * * @param gainFactor - Gain factor calculated using a known impedance. * * @param freqFunction - Select Repeat Frequency Sweep. * * @return impedance. ******************************************************************************/ double AD5933_CalculateImpedance(double gainFactor, char freqFunction) { signed short realData = 0; signed short imgData = 0; double magnitude = 0; double impedance = 0; int status = 0; // Repeat frequency sweep with last set parameters AD5933_SetRegisterValue(AD5933_REG_CONTROL_HB, AD5933_CONTROL_FUNCTION(freqFunction)| AD5933_CONTROL_RANGE(currentRange) | AD5933_CONTROL_PGA_GAIN(currentGain), 1); // Wait for data received to be valid while((status & AD5933_STAT_DATA_VALID) == 0) { status = AD5933_GetRegisterValue(AD5933_REG_STATUS,1); } // Get real and imaginary reg parts signed short RealPart = 0; signed short ImagPart = 0; unsigned char byte = 0; int tmp = 0; unsigned char registerAddress = AD5933_REG_REAL_DATA; for(byte = 0;byte < 2;byte ++) { // Read byte from specified registerAddress memory place tmp = wiringPiI2CReadReg8(i2cdevice,registerAddress); printf("\t\tReading from Register Address: 0x%02x...0x%02x\n",registerAddress,tmp); // Add this temporal value to our registerValue (remembering that // we are reading bytes that have location value, which means that // each measure we have we not only have to add it to the previous // register value but we also but do a bitwise shift (<< 8) by 1 byte RealPart = RealPart << 8; RealPart += tmp; // Update value from registerAddress to read next memory position byte registerAddress = registerAddress + 1; } registerAddress = AD5933_REG_IMAG_DATA; for(byte = 0;byte < 2;byte ++) { // Read byte from specified registerAddress memory place tmp = wiringPiI2CReadReg8(i2cdevice,registerAddress); printf("\t\tReading from Register Address: 0x%02x...0x%02x\n",registerAddress,tmp); // Add this temporal value to our registerValue (remembering that // we are reading bytes that have location value, which means that // each measure we have we not only have to add it to the previous // register value but we also but do a bitwise shift (<< 8) by 1 byte ImagPart = ImagPart << 8; ImagPart += tmp; // Update value from registerAddress to read next memory position byte registerAddress = registerAddress + 1; } magnitude = sqrt((RealPart * RealPart) + (ImagPart * ImagPart)); printf("Z = %hi + %hi*i ... |Z| = %f\n",RealPart,ImagPart,magnitude); return magnitude; } For those who aren't familiar with the pmodIA, in order to calculate the impedance you have to read the values of 4 registers: 2 of them to read the real part of the measure (0x94 and 0x95) and the other 2 to read the imaginary part of the measure (0x96 and 0x97). These values are signed, as explained in the datasheet. I realized that the original code I got from github stored these values into unsigned short variables and I changed that to signed short. Now the computation of the magnitude ( sqrt(Real² + Imag²) ) seems to be correct, as well as the values read from the registers. Using this code I can calculate the gainFactor. For the 12k resistor I used, at a frequency of 3 kHz, I get: Real = -79Imag = -35MAGNITUDE = 86.406021gainFactor = (1/ (MAGNITUDE * CALIBRATION_IMPEDANCE) ) = (1/(86.406021 * 12000) ) = 9.64439e-7 I use that gainFactor for all remaining impedance calculations. As an example of how my code seems to be working I copy/pasted the following lines (which are measurements of that 12k resistor at different frequencies). As explained before 0x94 & 0x95 registers contained the SIGNED value of the real part of the magnitude, while 0x96 & 0x97 registers contain the SIGNED value of the imaginary part of the magnitude. So, for instance, If I'm not mistaken, 0xffdf = -33, 0x0015 = 21. Reading from Register Address: 0x94...0xff Reading from Register Address: 0x95...0xdf Reading from Register Address: 0x96...0x00 Reading from Register Address: 0x97...0x15 Z = -33 + 21*i ... |Z| = 39.115214 Impedance read: 3067.859955 ohms (@ 767500 Hz) Reading from Register Address: 0x94...0xff Reading from Register Address: 0x95...0xde Reading from Register Address: 0x96...0x00 Reading from Register Address: 0x97...0x14 Z = -34 + 20*i ... |Z| = 39.446166 Impedance read: 3042.120759 ohms (@ 769000 Hz) Reading from Register Address: 0x94...0xff Reading from Register Address: 0x95...0xdb Reading from Register Address: 0x96...0x00 Reading from Register Address: 0x97...0x11 Z = -37 + 17*i ... |Z| = 40.718546 Impedance read: 2947.060034 ohms (@ 770500 Hz) Reading from Register Address: 0x94...0xff Reading from Register Address: 0x95...0xdf Reading from Register Address: 0x96...0x00 Reading from Register Address: 0x97...0x12 Z = -33 + 18*i ... |Z| = 37.589892 Impedance read: 3192.347538 ohms (@ 772000 Hz) Reading from Register Address: 0x94...0xff Reading from Register Address: 0x95...0xde Reading from Register Address: 0x96...0x00 Reading from Register Address: 0x97...0x12 Z = -34 + 18*i ... |Z| = 38.470768 Impedance read: 3119.251469 ohms (@ 773500 Hz) Reading from Register Address: 0x94...0xff Reading from Register Address: 0x95...0xdc Reading from Register Address: 0x96...0x00 Reading from Register Address: 0x97...0x11 Z = -36 + 17*i ... |Z| = 39.812058 Impedance read: 3014.162156 ohms (@ 775000 Hz) Reading from Register Address: 0x94...0xff Reading from Register Address: 0x95...0xde Reading from Register Address: 0x96...0x00 Reading from Register Address: 0x97...0x10 Z = -34 + 16*i ... |Z| = 37.576588 Extra: This is what happens when I put SEL to GND (same other conditions, and 12k resistor): Some extra information: I am supplying 3V3 to the pmodIA through the raspberry pi. How is it possible that I am getting these results? Is there something wrong with my code? Is my board broken?? Is maybe my settling time for the sweep configuration not right? Do I have to connect the SEL pin to GND or VCC? I would really appreciate your help as I need to finish this experiment in, at least, 1 month. Thank you so much for your attention, and let me know if you need more info. Manu B. Valentin
  3. @D@n, I marked this post as Urgent just to let anyone interested on answering know that it's not a DIY weekend project, but rather that it is important for my work. Sorry if that sounded like I considered this post anyhow more important than the rest of them. Today I spent the day looking for some inconsistencies or errors in the sign-bit (two complement) conversion as @D@n suggested. This is my updated code: pmodIA_test.tar. For those who aren't familiar with the pmodIA, in order to calculate the impedance you have to read the values of 4 registers: 2 of them to read the real part of the measure (0x94 and 0x95) and the other 2 to read the imaginary part of the measure (0x96 and 0x97). These values are signed, as explained in the datasheet. I realized that the original code stored these values into unsigned short variables and I changed that to signed short. Now the computation of the magnitude ( sqrt(Real² + Imag²) ) seems to be correct, as well as the values read from the registers. Now, I think that the problem I am facing here is related to the conversion between MAGNITUDE (sqrt (real² + imag²)) and IMPEDANCE. In the ad5933 datasheet they say this conversion is achieved by: IMPEDANCE = (1/ ( gainFactor * MAGNITUDE ) ) Where the gainFactor is calculated using the calibration resistor. I used a 12k resistor (following your advice @D@n) to see if the problem remained. Now, with a 12k resistor the response is not noisy, anymore, however it still shows values that I would never expect to obtain for a 12k resistor. For this case, the calibration returned: Real = -79 Imag = -35 MAGNITUDE = 86.406021 gainFactor = (1/ (MAGNITUDE * CALIBRATION_IMPEDANCE) ) = (1/(86.406021 * 12000) ) = 9.64439e-7 The following curve is the IMPEDANCE response for the 12k resistor. At low frequencies there are quite a lot of spikes and high values. Notice that this curve is actually pretty close to the 'open circuit' ones from my first question in this post. The following curve is the MAGNITUDE for that same case. These spikes are caused by the lower value of Magnitude at low frequencies. Now, if I zoom to a range from 15kHz and 1MHz you can see that it actually displays something more reasonable (impedance going from 12k to almost 4k). I repeated the test but now using the 200mVpp output range instead of the 2000mVpp I was using, and the noise appeared back (see following curve). Am I maybe saturating the ADC or something? @Natsu, thanks for your comment. Take a look at the curve I get when I put SEL to GND:
  4. Hello to everyone. I recently purchased an pmodIA board for a project we are conducting at work. The main goal is to obtain some resistivity vs frequency spectra for different objects as part of a object characterization problem involving Oil/Gas stuff. I already read the guide that digilent guys provide (http://store.digilentinc.com/pmod-ia-impedance-analyzer/) and I found some C libraries for the AD5933 (https://github.com/analogdevicesinc/no-OS/blob/master/Pmods/PmodIA/AD5933.c and https://github.com/analogdevicesinc/no-OS/blob/master/drivers/AD5933/AD5933.c). I am using a Raspberry Pi 3 Model B to control & communicate with the pmodIA. I based my code on those C libraries I mentioned. In order to use i2c protocol with the raspberry pi I use wiringPi library (http://wiringpi.com/reference/i2c-library/). In my code I have tested everything I've been able to, from all functionalities of the pmod: I get the temperature readings, set the frequency sweep params, make a freq. sweep measurement, etc. I have attached my code to this post (pmodIA_test.tar). All functionalities I tested seem to be okay, and everything seems to be working: I get the temperature, I can do a frequency sweep and if I connect the outputs of the pmod to an oscilloscope I can see the frequency sweeping (look at this video I recorded) Now, the thing is that even though everything seems to be WORKING, the result values I am getting back just make no sense AT ALL!!! For calibration purposes I am using a 100k resistor. In my code I first set the gain (X1) and range (2Vpp), I configure the sweep params and start it, then I calculate the gainfactor using that 100k resistor (which I later use on all my calculations), and then I ask the user to change the impedance and put the one that they desire to measure. On the main loop I simply do the frequency sweep, increasing the frequency at each step, calculate the impedance and put this value into an array (Z). I am also using gnuplot to display the results (the values obtained from the pmod are the exact same values that are being plotted using gnuplot, so this is not a display problem, as I checked this). This is when things get weird. These are the results I got for the following tests: 1) If I leave the 100k resistor for obtaining the impedance magnitude vs frequency I get the following curve ("100kresistor.png"). Just a noisy set of values, all of which are OVER 2 MEGAOHM. How is that possible, if I am using the same resistor I used to calibrate the pmod, that these results, for that same component, are so unaccurate? 2) (After the calibration with the 100k resistor)... If I place a 10uF capacitor in the connectors, I get the following curve ("Capacitor.png"). Now, the impedance of a capacitor should decrease with frequency according to 1/(2*pi*f*C). The curve I obtained, however, says otherwise. On the other side, the initial value of the impedance (2e+06) is almost the same as in the 100k resistor case, which is very suspicious. And here things get even weirder (at least to me). To connect the pmodIA with my protoboard I am using two SMA pigtails which carry the signals. 3) If I leave the circuit opened (using the pigtails) I get the following curve ("opencircuit.png"). This curve displays a peak at a certain frequency and then a decay (and at the end, it grows again). With an opencircuit I would expect to see, however, and +INF impedance, or a very high value (impedance of the air between the connectors). 4) If I take off the pigtails and repeat the same experience (open circuit) I get the following curve (opencircuit_NO_PIGTAILS.png). The results is almost identical, except that the values are lower. 5) If using the pigtails, I make a shortcircuit between the connectors, I get the following curve (shortcirtuit.png). This curve makes sense (at least a little bit, to me) as it is telling me that the pigtails are acting as inductors (because they are wires) and so that their impedance grows with frequency according to 2*pi*L. 6) If I take off the pigtails and shortcircuit the sma connectors in the pmodIA board directly I get the following curve (shortcircuit_NO_PIGTAILS.png). Again, like in the opencircuit case, the curve is almost identical to the one using pigtails, except that the values are a little bit lower. Some extra information: I am supplying 3V3 to the pmodIA through the raspberry pi. I am NOT connecting the SEL pin to anything (not GND nor VCC). How is it possible that I am getting these results? Is there something wrong with my code? Is my board broken?? Is maybe my settling time for the sweep configuration not right? Do I have to connect the SEL pin to GND or VCC? I would really appreciate your help as I need to finish this experiment in, at least, 1 month. Thank you so much for your attention, and let me know if you need more info. Manu B. Valentin