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DDC 112 gain adjustment

Other Parts Discussed in Thread: DDC112, DDC114

For DDC112, its mentioned that low amplitude signal such as photodiode, can be directly connected to analog input. But for such case how to increase gain by Range switches? Because by range swich, we add extra capacitance so voltage decreases that means gain reduces.

But if we want to increase gain internally or using Range switches , how it is possible?

  • Hello,

    I'm not too sure what you are asking but here are my thoughts.

    Yes the DDC112 analog inputs can be connected directly to a photodiode. The DDC112 is a current input device that is meant to be used with low-level currents such as photodiodes. The RANGE0, RANGE1, and RANGE2 pins are used to select the internal or external capacitor that the device feedback network will use. Increasing the capacitance on the feedback capacitor will increase the range of the device since it will be able to integrate more charge given a higher value capacitor. Looking at page 9 of the DDC112 datasheet, you can see Figure 3 which shows the internal block diagram. Looking at Table 1 you can see how the RANGE switches will affect the capacitor selection and therefore change the charge range of the device. At the beginning of the charge integration cycle, the feedback capacitors are charged to the chip VREF level. The input current during integration will remove some of this charge. After the integration cycle, the converter after the input stage will compare the capacitor charge to the original VREF charge value and a resultant input amount can be calculated.

    Please let me know if I have answered your questions.

    Regards,

    -Adam
  • Thanks for detailed answer..but I am few queries....

    1. increasing range means exactly what....increasing analog input voltage amplitude or vice versa.. bcoz in integrator, vo =vin . T / R.C so increasing Capacitance through range selection means decreasing Vo as per equation...

    2. What is the max input voltage it takes at input analog pin...In data sheet I got analog i/p current is 150 uA max,, but voltage is not specified.

    Because when I am connecting 3V at input analog pin, its getting hot little bit.

    3.Can it sense photodiode low range signal ( in the range of nA) with precision.

    Pls suggest.

    Thanks in advance.

  • Hello,

    1. As I mentioned before, the DDC112 collects charge over time. Increasing the range means that the device can collect more charge (either the same current for a longer time, or a larger current over the same time). For example, range 1 has a max range of 50pC, meaning a 100nA current can be integrated for 500uS. Range 7 has a max range of 350pC. This means the same 100nA current can be integrated for up to 3500uS or 7 times more. A current of 700nA can also be integrated for 500uS in range 7. You don't need to worry about the Vout since it is being compared internal to the chip. You will never see this Vout, you will only see the resulting change in output code produced by the device.

    2. The voltage is not specified because the device is not a voltage input device. You need to find out the range of current you will be inputting to the device and multiply it by the integration time you are using (the integration time is set by the CONV frequency). Input charge = integration time * input current. The device range 1-7 must be selected so that you have enough capacity to integrate the input charge you are looking for. For example, to integrate a 100nA current for 1000uS, you need at least 100nA*1000uS or minimum 100pC range.

    You cannot connect a straight DC voltage to the input pins, there is no resistance there and therefore no resulting current can be calculated. In our test setup, we use 10MegOhm resistors in series with the inputs so that we can calculate the resulting current. The other option is to connect a photodiode or similar to the input that when excited will produce a known current.

    3. Yes it can. The device is designed to integrate current directly from photodiodes with pA or nA precision.

    Regards,

    -Adam
  • Thanks for answer...
    I have two specific queries which I am facing in my ambitious and challenging project.
    In my application, I am using 650nm, 6mm, 3V, 5mW Laser Dot Diode as a light source and photodiode is S1133-01 ( dark current : 10 pA )
    Link for datasheet: http://www.hamamatsu.com/resources/pdf/ssd/s1087_etc_kspd1039e.pdf

    Basically my application is bio chemistry analyzer, in which concentration of any compound is found out by passing light (from 650nm, 6mm, 3V, 5mW Laser Dot Diode) through liquid sample... Sample will absorb some amount of it and will transmit some light and then remaining light will fall on two photodetectors ( S1133-01 both)...one is placed straight to light source after liquid sample and second is at 45 to light source°. And photodiodes are connected to in1 and in2 respectively.

    Photodiode which is placed straight is what we call "Transmission detector".
    And photodiode which is placed at 45°is "Scattered photodetector'.

    Pls note:
    in1: Transmission detector
    in2: Scattered detector

    1. Can this ADC will sense this light ( flow: Laser---> S1133-01 photodiode---> DDC112 analog input in1 ) for Transmission detector.
    2.For second analog input in2, I have connected scattered light from laser, so this light strength will be less than transmission detector. Can it sense, this very very low strength signal.

    Waiting for ur reply.

    Thanks in advance.
  • Hello,

    Yes I think this will work well. The DDC112 does have pA precision so a dark current of 10pA can be measured. What are you expecting your maximum photodiode current to be? You will need to select the correct DDC112 range and CONV time so that you do not saturate the DDC112 inputs too easily. Saturating the inputs will not damage the device but you obviously cannot measure usable data if the inputs are saturated. 

    As for your questions 1 and 2, Yes this should work.

    Regards,

    -Adam

  • Thanks Adam for reply.

    In datasheet, S1133-01 has mentioned short circuit current as 5.6 uA. Is this max photodiode current?

    And if max current is 5.6uA , what will be arrangement for range swithces? Range3, 2, 1 will be grounded in this case?

    And does position of photodiode matters that much. Because, for Transmission detector, photodiode is mounted on another PCB which is very nearby ( apprx. 3 cm distance ) and through 3 cm wire connection is established . Is there any noise or false sensing problem?

    And if TEST, CONV, CLK, DCLK, DXMIT, DVALID and  Dout pins of DDC112 are on another board  ( distance pin to pin 3cm )and using connector its coming to these DDC pins, in this case....Is there any noise issue...On DDC board, there will be analog and digital plane. Please suggest because I have to keep design noise proof and measurement should be precise since blood samples are tested with this instruments and reading must be correct.


    Thanks once again.

  • Texas team,

    I am waiting for your valuable reply.

    Thanks.

    -Vaibhav

  • Hello,

    Max current of 5.6uA is too high for this device, you will over saturate the inputs by more than 2 times. Basically the minimum integration time is 500uS, the maximum range with external capacitance of 250pF and Range2/1/0 all grounded is 1000pC. 1000pC/500uS gives us 2uA. This 2uA is the highest saturation current available in Continuous mode. I highly recommend continuous mode since it is a lot easier to use. You should try measuring the output current of the diodes given your setup and let us know. Given that you won't be completely saturating the diode, you may not have 5.6uA or even 2uA of current. 

    Our customers usually use an arrangement like your where there is some distance between the diodes, DDC chips, or FPGA controller board. This should not affect your results as long as you do not have a large amount of capacitance added on your input traces. As for the control signals, just make sure you monitor these signals for excessive ringing or other signal integrity issues and adjust accordingly. 

    Regards,

    -Adam

  • I have selected 1.3 uA photodiode now to avoid conflict at transmission detector where signal strength is more and 5uA photodiode at scattering detector as scattered signal will be having less strength and photosensitive area is more in this diode. ( though 5 uA diode signal will be less compare to rated Isc value and second reason for selecting this diode is because its Photosensitive area is more which is good if scattered signal strength is less  )

    What will be good practice for PCB layout. In my PCB, CLK,DCLK,TEST,DXMIT,Dout.CONV these pins are from another board. Will this cause problem?

    Pls suggest good pcb layout scheme according to your experience.

    Thanks.

  • Hello,

    The signals you mentioned should be fine over a few inches with no issues. You might leave options for a series resistor and a capacitor to ground on each of these signals so that you can adjust the termination/filtering if signal integrity issues occur. Even if you don't need them in the end, this is a good option, and you can always leave the capacitors uninstalled as well as use a 0 ohm jumper resistor in place of the series resistance. We have sent some most of these same signals over several meters in the past with appropriate termination resistance without issues. 

    Layout suggestions are given in the DDC11X EVM User's guide (http://www.ti.com/lit/ug/slau234a/slau234a.pdfwhich supports the DDC112 and DDC114. Please see section 3 for board layout suggestions. Section 8 also shows complete board design, gerbers, and schematics for both the DDC112, DDC114, and connecting motherboard which houses the FPGA and USB circuitry.

    You can also see some layout guidance in the DDC112 datasheet (http://www.ti.com/lit/ds/sbas085b/sbas085b.pdfin the Layout section.  

    Regards,

    -Adam

  • Vaibhav,

    I wanted to check in to see how your project is going. Are you in mass production? If not, when do you plan to?