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Limitations of Measuring Small Currents

May 7, 2017, 8:00 pm
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This is a question for Marc Ramos, a followup to his answer of March 16 regarding a question about measurement limitations in amperometry.  I too have recently laid out and fabricated a custom ADuCM350 board to measure very small currents.  However, I have problems in getting accurate results (i.e. I don't measure 32768 when amperometric.c applies 0 V across the load, in my case a 4 Mohm load).

 

Marc,

   You posted a schematic that showed using the ADuCM350 with an RTIA of 12 Mohms.  Did you really get that to work?  What value did you use for your RCAL resistor and what values for RCAL/RTIA did you type into your code?  That is, did you type in "12000000" into your code in the call in amperometric.c that asks for the value of RTIA, or did you measure the resistance after you soldered in the 12 Mohm resistor and enter the actual measured value of RTIA when it was in parallel with the ADuCM350?  

 

    The reason I ask, is that if I measure the value of my 3 Mohm 0.1% resistor in series with a 1 Mohm 0.1% resistor before soldering them into the pads for RTIA, I measure 3.9997 Mohms.  Fine.  However, once they are in the board, in parallel with the ADuCM350, I measure 1.22 Mohms.  So what would you type in for RTIA (i.e. does the api's error correction take care of this discrepancy)?

 

     A similar issue arises for RCAL.  If you remove the RCAL resistor and measure the input resistance looking into the ADuCM350 from RCAL's pads, you measure 1.54 Mohms.  I tried all sorts of values for RCAL, and all possible combinations of typing the measured in- vs. out-of-the board values into the place where amperometric.c asks for the value of RCAL, but I still never get the result of 32768 when 0V is applied across the 4 Mohm load.

 

    It seems then, that there are definite limitations of using the ADuCM350 for measurement of very small currents.  Would you concur?  What's the smallest increment of current you've been able to resolve?

 

    Or is the problem that one really needs a Keithley 2401 to apply an accurate 1.1V (for zero volts across the load)?  That is, is the problem with the ADuCM350's DAC not being accurate enough?

 

    What is the status of your new board for measuring small currents?  Have you run into similar problems?

 

Thank you.

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Table 547 mistake?

May 15, 2017, 11:31 pm
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Hi, I'm trying to understand the calibration routines in amperometric.c:  adi_AFE_TiaChanCal() and adi_AFE_ExciteChanCalNoAtten().  In studying the No Factory Calibration section of the UG-587, the Hardware Reference Manual, I'm wondering if there aren't some mistakes/typos?  My questions are detailed in the attached doc.  Why is the cal open state in the first row of Table 547 set to 0x3000?  Shouldn't it be 0x0000?

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Noise-free Resolution of ADuCM350

May 23, 2017, 10:06 pm
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Mark,

     On your new board where you've worked hard to keep the noise low, what do you get for the ADuCM350's noise-free resolution (there's no such spec listed in the datasheet).  That is, for 160,000 samples/sec what is the effective number of bits (or noise-free resolution)?  I'm getting about 12 bits, see attached, when I do the measurement for ADC offset (VBIAS against VBIAS).  At 900 SPS, there's zero noise, but at 160,000 SPS, I get ~20 counts peak to peak noise.  Is

this what you see?

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TEMP_SENS_N to TEMP_SENS_N measurement

May 27, 2017, 11:43 pm
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Hi,

   I have a question about the measurement for MUX_SEL = 0b11010, where TEMP_SENS_N is measured against TEMP_SENS_N (i.e. to measure an offset of the measurement channel).  How is this different than the measurement for MUX_SEL = 0b11000, which measures VBIAS against VBIAS?

 

    I noticed that in the 4-wire BioImpedance example's calibration of the current measurement, it does *both* of these measurements (don't they give the same answer)?

 

     Even weirder, the 4-wire BioImpedance example uses the result of the TEMP_SENS_N to TEMP_SENSE_N measurement to set the value in the ADC_OFFSET_TIA calibration register.  What would the measurement of TEMP_SENS_N to TEMP_SENSE_N have to do with the offset of the TIA?  I'm attaching some notes to make my question clearer.

 

Thank you.

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Precision References and RCAL

May 30, 2017, 9:37 pm
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Hello, I have a question about a section in the Hardware Reference Manual (UG-587, p. 390) entitled "Precision Voltage Reference vs. RCAL".  That entire section contains only two sentences ... and two rather mysterious sentences at that.

 

Those two sentences claim that the choice of RCAL somehow affects the performance of the precision references.  However, the figure on the previous page, which shows the schematic for the precision references, shows RCAL is not involved there.  

 

How can the performance of the references (transient response, load regulation etc.) by affected by RCAL?  Is some documentation missing?  I've attached notes to clarify my question.

 

Thank you.

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HMC996LP4E Vctrl

June 8, 2017, 11:39 am
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Hi EngineerZone

I'm designing the HMC996 into my receiver and would like to know if there is recommended circuitry for driving the gain control Vctrl pin, and be able to vary it digitally ?

Many thanks in advance.

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Need Guidance on DAQ2 NoOS Sample Rate.

May 24, 2017, 8:30 am
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I see a lot of confusion regarding changing the sample rate of the DAQ2 board:  My eyes are bleeding after reading all of these threads.

Changing the sample rate on the FMCDAQ2 board 

problem with JESD to DAC on FMCDAQ2 using no-OS 

How do you reduce the effective sampling rate of the DAQ2 noOS project for the KCU105 with analog JESD204B? 

Can you please guide me on the easiest NoOS code base, and HDL base and EVERYTHING I need to do in order to change the sample rate?

I am using KCU105, DAQ2, NoOS drivers  and Vivado 2016.2, but I prefer 2016.4.

    I am looking to set the sample rate = 250Msps for the DAC and ADC.  

 

Thanks 

Jade

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Why is the Libiio Simulink block for the AD9361 fixed in minor steps (FiM) e why doesn't it work with discrete (D) blocks?

June 8, 2017, 11:43 am
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Why is the libIIO Simulink block for the AD9361 fixed in minor steps (FiM) e why doesn't it work with discrete (D) blocks? I can't find any reasonable explanations or solutions to this problem.

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the disturb of AD8237

May 31, 2017, 8:22 pm
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first,the output of AD8237 has the disturb of 50hz,When no difference signal input,but when the difference signal inputs is applied to the pin 2 and 3 of AD8237,this is ok. the disturb signal is as follows:the disturb of AD8237 output

 

secondly,i want to introduce my project application background.the Wheatstone bridge is used to test the vibration ,but differential signals have high frequency part caused by vibration,which make the power of AD8237 have more noise,for example ,when the Wheatstone bridge is excited by vibrating,the power of AD8237 have more noise,may be the transient voltage will be above 6v.

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Use ADALM1000 to record 30 seconds length of data

June 7, 2017, 10:01 pm
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Hi, Dave and all ADALM1K experts,

 

I plan to use adalm1000 as DQA device. 

The board has fixed sample rate of 100k SPS, with buffer size about 100k records, the total length of data is 1 second. 

I wonder what is the best way to pull out the data of the board continuously and reliably with minimum effort. The Application I would like to apply would have up to 30 seconds duration. 

I also like to display/replay the whole length of data with ALICE.

Another question is about time stamps. I wonder if I can add accurate time stamp along with Voltage record on CA and CB, e.g. by using of RTC on the ATSAM3U1C? 

I understand the current data record is indexed with fixed time interval of 10 microseconds, based on 100k SPS of the ADC. 

I am new to Python, any example would be helpful. 

Thanks

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ADF4356: Not able to change output frequency

June 8, 2017, 1:12 pm
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Hi there,

 

I'm using 3.3V Arduini Pro Mini to program adf4356 through SPI interface. I'm getting a fixed 4.6GHz signal from the chip and I'm able to change the output power level of the chip among four different levels. However, I can't change the output frequency by changing the 10 bit R counter value or integer value (I'm using adf4356 as an integer-N PLL). Does anyone know what might be the reason? Any suggestion or reply will be appreciated.

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AD8318 EV kits

June 8, 2017, 1:14 pm
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What is the difference between the two EV kits listed for the AD8318?

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ADAU1701 Interface register read value incorrect

June 2, 2017, 6:38 am
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Hi Guys,

 

I am planning to use 6 interface registers to store the ouput data of 6 RMS tables for an Arduino to read through I2C. When reading the data with the test setup shown below the data was shifted and an extra character was added at the end. When I read the register with the read/write window the same thing happens and is also shown below. 

 

Why is this happening and how do I get the correct data to show up, as shown in the 2nd DSP readback from the left? Other register values seem to be read right. I can for example find the exact value shown in the 1st DSP readback.

 

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ADAU1701 how to use the data capture registers

June 2, 2017, 6:50 am
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Hi All,

 

I have a few questions regarding the 2 byte Data Capture 0 and 1 registers at adress 1x081a and 0x081B. I can't seem to find alot of information about these registers at all.

 

How much data can be stored in each register and how do I route data to the registers in Sigma Studio? Can I store more than one value at once in each register? Any more useful information would also be welcome.

 

If possible I would like to store at least 3 RMS table output values in each Data capture register, which will be read by an Arduino.

 

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FSCOMMS2 ADC output delay and TX_MON path

June 8, 2017, 3:21 pm
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Hi,

 

I have recently been evaluating FSCOMMS2 for a project and have trouble figuring out some of the timing specs of the HDL blocks used in the Vivado project on the Github. Specifically, I am wondering what is the timing relation between the output of the axi_ad9361 IP (adc_data_i0, adc_data_q0, adc_data_i1, adc_data_q1) with repsect to the LVDS input? Since the l_clk is derived from the LVDS clock, is adc_data_i0 going to be 1  or 2 clock cycles delayed from the LVDS input (rising edge of rx_frame_p)?

 

My second question is about the TX_MON path. I am trying to get the TPM Test Mode working. So I can receive I/Q data at the RX output. How should I do that? Which RX port is it going to use?

 

Thanks,

Tao

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Maximum Pull up voltage for PDOs of ADM1166 while used as a Open Drain Output

July 20, 2016, 4:09 am
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Hi,

     I am planning to use PDO pins as Open drain output in my application. Kindly suggest the maximum voltage that the pin can be pulled up in Open drain mode. The datasheet claims the absolute maximum rating of the pin is 16V. Any how i have planned to pull up to +15V.

Regards

Surya

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Inside of ADG 407.

June 7, 2017, 5:57 pm
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It is a somewhat strange question about ADG 407.
A voltage of about 2 V was observed at the switch output when logic input was applied while power was not applied.
What is the path of this voltage appearing?

 

 

Best regards

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AD7864 input terminal

June 7, 2017, 6:06 pm
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It is a question about AD7864.
The absolute maximum rating of the input terminal of the AD7864 is written as "-7 V to + 20 V".
Is this applicable even when the power is off?

Best regards

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ADG3301 ONE-SHOT GENERATOR

May 30, 2017, 9:02 pm
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Hi,
When A pin is connected to GND with low impedance and the data singnal is input to Y pin with low impedance, How does one-shot generator work? I'd like to know the risk, confirm T1,2,3,4 doesn't on and large current don't flow on this irregular situation. I guess large current doesn't flow because of 6k ohm register at stable condition. But I don't know about this situation. 

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Seeking Simple FM Modulator/Demodulator

September 6, 2014, 5:32 pm
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(note, i submitted this to Analog Devices tech support; also posting here for extra suggestions. thx!)

 

We are a 501c3 federal nonprofit organization. Our mission is to mentor disadvantaged youths in electronics fabrication and design. Seeking a semiconductor chip or module that meets the specs below:

 

General Description: A simple analog RF transceiver/modulator/demodulator (or transmitter/receiver chip-set). Simple frequency modulation  is all i need, with an output suitable for driving an antenna.

 

Modulation Input: The Modulation input will be a variable analog signal, in the range of 100 Hz up to 10 Khz.

 

VCO:

    -Transmitting in the range of 900 Mhz.

    -Integrated on-chip VCO.

 

Stability: Need PLL stability

 

Modulation bandwidth: The modulation-bandwidth should be equivalent to at least 16 bits of resolution (ie comparing the pre-modulation analog input with post demodulation analog output-- if that makes any sense.).

 

Analog/Digital: Simple analog end-to-end is fine; does not have to be a digital product. (External MCU control is a nice-to-have, but not required).

 

Seeking a low-cost, low external parts count, complete RF transceiver solution.

 

End product: The end product is a portable consumer audio device.

 

thx!

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