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LMH7322: Design query

Ashish Kumar Pandey
Prodigy 165 points

Part Number: LMH7322

Tool/software:

Hi


We are using LMH7322 comparator to compare two analog inputs in one of our new design.
The LMH7322 output will be LVDS signal which will be interfaced with a 2.5V LVDS receiver.
As termination requirement is not clear from the datasheet, please suggest the suitable terminations required at both input and output end of LMH7322.

Voltages used in LMH7322 are as follows:
VCCI = +5V
VEE  = -5V
VCCO = +2.5V

Block diagram is attached for your reference

Regards,

Ashish

  • +1
    Guru 321460 points

    The LMH7322 itself does not care.

    The LVDS line should be terminated like any other LVDS line, i.e., with a resistor that matches the characteristic impedance of the line, typically 100 Ω.

    The analog inputs need to be terminated if its signal edges are so fast that you get transmission line effects.

  • 0 in reply to Clemens Ladisch
    TI__Genius 13305 points

    Hi Ashish,

    Please read page 12 of the data sheet:

    The output stage of the LMH7322 consists of two emitter followers; the output transistors will try to source however much current it needs to across the output resistors to maintain a swing of VCCO - 1.1V to VCCO - 1.5V. By setting VCCO to 2.5V, you are mimicking an LVDS output since each output swings from VCCO - 1.1V and VCCO - 1.5V (1.4V to 1.1V) . For correct biasing of the output stage, the termination voltage needs to be at least VCCO - 2V. Suitable terminations are dependent on your power requirements. Since the outputs are emitter followers, it will constantly be sourcing current which will be dissipated across the termination resistor. As stated in the screenshot section, the tradeoff between using a higher valued termination resistor is that the power consumption goes down but the output stage transition slows down.