Post Go back to editing

LT1711

Hi

I have a question LT1711.

· About 10 VTH and VS in the data sheet
Is VS the potential difference between + IN and - IN of the LT1711?
(If VS is 3 V, + IN is + 1.5 V and - IN is -1.5 V?)

Is VTH the voltage actually applied to the input terminal of the LT1711?
For example, if VS is 3 V and data sheet constant is implemented
Since it is stated that the attenuation amount will be 0.0978, for the input signal 3 V
Values recognized by the comparator as input signals are 3 V x 0.0978 = 0.2934 V
Is it that?

About 240 / 750k =. 03% in original text. With 3 V drive
Is 240 a value twice that of R 3? Or Z 0 = 120 Ω in the data sheet
Is it twice the value of Z0?
In addition, 3 V drive is not the above VS but the input supply voltage of the comparator
(3 V in the data sheet figure) is it?

-------------------------

The hysteresis is very small, because the large 750k feedback resistor looks back into the relatively low impedance of R3 looking back into the 120 Ohm line.

That impedance is about 240 Ohms, so the hysteresis as a percentage is 240/750k = .03%.  With 3V drive, that is 1mV.  But it is applied twice, so total hysteresis is 2mV.

Hysteresis could be increased by reducing the 750k resistors.  Note that the full scale signal at the receiver inputs is only 700mV differential, because of the attenuation of the resistor networks.

The threshold is 0V, differential, over a fairly large valid CM range.  (But of course hysteresis sets each real trip point a little bit away from that.)

------------------

Best Regards

HOD

• Hi HOD:  I wrote up that application, so I can help with it.

Vs is supposed to be the total supply voltage assuming a rail-to-rail output, so for that circuit it would be 3V.  However, a better number to use would be the de-rated voltage knowing that the output voltage is not truly rail to rail, but has some losses.  For the LT1711, looking at the true output swing specs into 1mA load, Vs would be 2.6V as 0.2V is typically lost on each swing. Normally, Vs is Vsupply, but for the purposes of the signal calculations it should really be Vswing.

Vth is not Vthresh but rather Vthevenin.  It is the unloaded Thevenin equivalent voltage presented by each driver across its local Ro, before it is loaded by the line.  When loaded into a matched line, the actual voltage will be half of that (and will of course be modulated by the other driver).

Sorry for not including strict definitions of abbreviations in the text itself.

• Hi

Is Vswing the output voltage?
Also, if the power supply voltage is 3 V but the output voltage is 2.6 V, is it necessary to calculate VTH with Vs = 2.6?

Is it actual voltage = Vthevenin?
Or does it mean actual voltage = Vthevenin / 2?