# How to determine LV CMOS output impedance?

I've successfully been using this circuit:

to generate a +13 dBm sine output (the frequency is 10 MHz). The NB3N551 has an output impedance spec of ~20Ω or so, which means the input to the pi filter matches.

I've been trying to do the same thing, but instead of the NB3N551 using a 74LVC2G04W6-7 output. I've tried varying R1, but the output doesn't look at all like a sine wave. It looks more like a square wave, but with slower rise and fall.

What is the 74LVC2G04W6-7's output impedance (Vcc is 5v)? How can I figure that out from the datasheet? Is it just too high for this purpose?

• Voltage drop over output current. – Ignacio Vazquez-Abrams Apr 1 '18 at 15:55
• There's a line in a datasheet table that says Voh @ Io -32 mA = 3.8v with Vcc 4.5v. Does that imply an output impedance of 21Ω? – nsayer Apr 1 '18 at 16:05
• That it does... – Ignacio Vazquez-Abrams Apr 1 '18 at 16:05
• Well, then I don't understand why the LPF isn't generating a nice clean sine wave in this case, but I guess that's another question. – nsayer Apr 1 '18 at 16:06
• Try switching to the LVC2UG04 so you don't have to deal with the buffering. – Ignacio Vazquez-Abrams Apr 1 '18 at 16:07

NB3N551

Voh Output HIGH Voltage @ Ioh = −25 mA, Vdd = 3.3 V , Voh=2.4 V (max)

(3.3-2.4)/25mA = 0.9V/25m = Ron= 36 Ω MAX (25 Ω nom is normal) for Nch

Vol Output LOW Voltage @ Iol = 25 mA, Vol= 0.4 V (max)

0.4V/25mA= Ron= 16 Ω (max) for Pch

74LVC2G04W6

Ioh= 2.3 V @Vdd=3V @ -24mA @25'C Ron= (3-2.4)/24mA = 25 Ω

Computing ESR of a logic driver is easy but it is not always symmetrical.

Since these parts are similar technology with ESR in the 25 Ω nom average range, (74LVC family) and you have neglected to specify f, load Z and reported excessive harmonics from the "squarish" slew response I can only surmise you have not used the same frequency for this 12MHz 3rd order 50 Ohm LPF.

With no load you will get +4.1 dB peaking at 12MHz then -6dB per octave above 14Mhz.

With a 50 Ohm load then you can use dBm readings and expect -6dB / octave attenuation above 12MHz.

So if a 5Vpp signal square wave with a 50 Ohm source intoa 50 Ohm load can generate 3.175V pp fundamental or +23 dBm at 12MHz to get +13dBm your f would have to be ~20MHz with a 50 Ohm load.

Or if you wanted higher attenuation of harmonics and greater output power with a tradeoff for sensitivity from higher Q,...

... In theory if had the lowest ESR driver (Ron) then did not add Rs=x Ohms the series Q could be Q= ~30dB in gain at 10MHz with no load. Then if you tested it into a 50 Ohm load ... and Rs=0 , it becomes flat LPF response with 0 dB loss < fo ) , but then input Cap is shunted with Rs=0 so you end up with a 2nd order LPF filter instread of a 3rd order filter. So no gain in harmonic suppression.

This is why Rs must match filter breakpoint impedance and load for stable LPF or for just generating a sine wave, at the risk of high sensitivity to Q and LC values affecting signal level with Rs=0 and no load by high voltage gain (dBV ) but not power gain dBm since there is no power in the reactive amplification.

• The frequency is 10 MHz. With the NB3N551 (Vcc 5v), this circuit works quite well. – nsayer Apr 1 '18 at 21:19