• State Resolved
  • Locked Locked
  • Replies 1 reply
  • Subscribers 46 subscribers
  • Views 253 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADC08351 Questions

Michael Raleigh
Expert 3245 points
Other Parts Discussed in Thread: ADC08351

Hello,

I have a few questions about the ADC08351:

1.) How sensitive is V_A? Is it required to use a dedicated power source if there is only a single source on the board? Can it share the power supple with other devices? eg pulse laser, image sensor, etc ?

2.) Is there any offset to the Vp-p f? When I use a 2V power supply I get 2.04Vp-p as per the datasheet. However if my signal is 0.7V - 2.7V, is there anyway to achieve a 0.7V offset on the ADC Vp-p so that it can stay within the 2V Vp-p range but really range from 0.7V to 2.74V ?

3.) What is the max distance between the ADC input and the signal source that can be had WITHOUT a buffer?

Thanks!

  • 0
    TI__Expert 4340 points

    Michael,

    1) Please assume that the ADC requires a low noise power supply. It may be possible to share supplies with other analog signal path devices, though ferrite isolation is recommended. I would not recommend sharing supplies with other digital devices.

    2) I assume you mean VREF=VA=3V results in a 2.04 Vpp input range (and not 2V a supply). In this case, the center point of the input range is (0.0665+0.75)/2*VREF=1.225V and the range [min, max] = [0.2, 2.25] V. It appears that your 0.7-2.7V signal would require a ~0.5V negative offset to fit in the range. There does not appear to be options to internally shift the reference in the ADC by an offset. Here are some options:

    - AC coupling. The ADC has internal bias resistors that correctly sets the center voltage for AC coupled applications

    - Resistor divider. Use a divider to scale the center voltage of your signal (1.7V) down to the center point of the ADC (1.225V) relative to GND. This would also reduce the signal swing by 28% (-2.8dB).

    - Amplifier level shifter: A solution similar to the circuit shown in Figure 24 of the datasheet could both shift the center votlage and maintain the 2Vpp signal but at the cost of added noise and distortion from the amplifier.

    3) The max distance depends on the frequency content of your signal and the acceptable VSWR at those frequencies, as well as the amount of acceptable loss through the long transmission channel. Generally, for long distance applications, more design attention must be paid to matching the transmission line to the load (ADC input in this case). An external resistive termination at the input to the ADC is likely required and perhaps some extra components for an impedance matching network. Matching the Signal source to the transmission channel would also be recommended because the ADC has charge kickback that travels across the channel in the reverse direction. Obviously, if the channel attenuates the signal too much, an amplifier would be recommended to boost the signal before it gets to the ADC. In general, I would recommend putting a buffer between the long channel and the ADC which will allow you to boost the signal after channel loss, achieve good impedance matching with the channel, and avoid the somewhat unpredictable effects of charge injection from the ADC in the channel.

    As a rule of thumb: If the distance between the signal source and the ADC input is longer than ~1/10th the wavelength of your highest frequency, then you must seriously consider the transmission line effects of the channel and use the above recommendations to maximize performance. 

    Regards, Josh