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ISO3088: Differential input circuit and minimum signal levels

Mark Peffley
Prodigy 110 points
Part Number: ISO3088
Other Parts Discussed in Thread: ISO1410

Tool/software:

Hello, I would like to understand the input requirements of the ISO3088 A little better. Figure 6-10 shows minimum recommended Differential Input Voltage vs Signal Rate. I am running at 10Mbps so it recommends a differential voltage of about 650mV.

Say I have a square wave as the input signal. Does graph 6-10 mean the voltage from the top of my square wave to the base of my square wave should be 650mV total swing or does it require +650mV to -650mV. I have seen the definitions of Vid in Figure 7-7.  There is no accompanying waveform so I am unclear.


If the input  hysteresis thresholds are all between -200mV and -10mV (Table 6.9), why does Fig 6-10 recommend a 600mV  swing all the way down to DC?   The A/B input schematics in 8.4.1 don't look to me like a traditional differential amplifer (unless that emitter current source is shared between the inputs).  Is it possible to share a more detailed schematic of the differential input section?


  • 0
    TI__Mastermind 28385 points

    Hello Mark, 

    Good questions. The A and B pins only need to have a differential voltage that passes VIT+ or VIT- to change state. As you showed in table 6.9 this is -200mV to -10mV. The recommendation of VID > 600mV is to allow for headroom to improve signal integrity. Without this margin, small fluctuations in the differential signal could cause missed bits and data errors.

    Unfortunately, I cannot share further details than what is shown in the device I/O schematics. However, please understand that this device is not a differential amplifier and the ISO3088 is a RS-485 transceiver and is intended to detect a differential digital signal. 

    For your reference, please see the ISO1410 which has clear waveform for typical operation of a RS-485 transceiver. 

    Best,
    Andrew

  • 0
    Prodigy 110 points

    So to frame it in my original terms, I should aim for a swing of +650mV to -650mV. Vid is an instantaneous measurement, not a waveform level measurement. For design purposes, the most relevant guideline is the recommended signal level figures and not the VTH+ / VTH- / VHYS spec.


    www.ti.com/.../iso1410.pdf page 27
    Equation 2: VID = VA – VB

    --The receiver is enabled when the receiver enable pin, RE, is logic low.

    --The receiver output, R, goes high when the differential input voltage defined by Equation 2 is greater than the positive input threshold, VTH+.

    --The receiver output, R, goes low when the differential input voltage defined by Equation 2 is less than the negative input threshold, VTH–.

    --If the VID voltage is between the VTH+ and VTH– thresholds, the output is indeterminate.

  • 0 in reply to Mark Peffley
    TI__Mastermind 28385 points
    Mark Peffley said:

    Equation 2: VID = VA – VB

    --The receiver is enabled when the receiver enable pin, RE, is logic low.

    --The receiver output, R, goes high when the differential input voltage defined by Equation 2 is greater than the positive input threshold, VTH+.

    --The receiver output, R, goes low when the differential input voltage defined by Equation 2 is less than the negative input threshold, VTH–.

    --If the VID voltage is between the VTH+ and VTH– thresholds, the output is indeterminate.

    Yes. Everything here is correct. 

    Mark Peffley said:
    I should aim for a swing of +650mV to -650mV.

    Correct, +/-650mV is the recommended minimum differential swing for the receiver and signal integrity. As you have mentioned, are the VTH+ / VTH- / VHYS specs are design requirements that describe where the device will actually switch.

    Best,
    Andrew