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Re: Level Shifer

Tue Nov 10, 2015 7:22 pm

Generally only more modern SMD FET's have such low gate voltages, pin through hole versions normally do not, but maybe there are exceptions.

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Re: Level Shifer

Tue Nov 10, 2015 10:13 pm

According to LTSpice circuit does not work between 2.7 V and 3.3 V with BS170. The gate voltage seems to be critical and 3.0 V just at the lower limit.

Yes there are other FETs with low Vgs(ON) and few of them are in TO-92 package. Nice video on testing FETs:

In LTSpice model the circuit seems to work at low frequency between 2.7 V and 3.3 V if I take the gate bias from high side or 3.3 V and use 100 kohm between gate and source. To be tested.

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Re: Level Shifer

Wed Nov 11, 2015 2:51 pm

anyone can tell me the difference of 74HC4050N from nxp and CD74HC4050E from TI
the first cost 0.85 € and the second 0.35 €

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Re: Level Shifer

Wed Nov 11, 2015 6:58 pm

Two links about level shifters ... y-voltages ... sBrchr.pdf

And a video, does not take into account parasitic reactance, like input capacitance, resistor inductance, oscilloscope input capacitance, breadboard capacitance, etc.

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Re: Level Shifer

Thu Nov 12, 2015 1:17 pm

After read some text, and watch videos about voltage level shifter, and many of these text or video recommend to avoid voltage divider for high frequency, I decide to simulate some circuits to check that happens.
We need some information before start design a level shifter, as operation frequency, input capacitance, input resistance, input inductance, and output impedance, normally very low. And others.
I not found RPi electrical characteristics.
Critical information is input capacitance, similar devices have near 5pF per pin. I used three times this values, or 15pF.
Input resistance is very high, 500kOhms or more. And low inductance. Both not take on account.
As simulation software I used NI Multisim 11.0 (windows XP).
I tested two version of voltage divider, one at 10mA and other for 1mA.
C1 and C2 is a input capacitance.
R1 and R2 is 10mA divider, and R3, R4 is 1mA divider.
Signal is 10MHz.
10mA is usable on most applications.
1mA, can be not if need to measure time on high or low state.
Same circuit at 100MHz
1mA is useless.
10mA can be used on some situations.

Inserting a compensation capacitor parallel to R3 or R1.
Now, 1mA version is usable at 10MHz, better that 10mA version.
1mA version at 100MHz, without compensation and with.
Compensated signal is very nice.

Test Square wave signal have a 1ns of rise/fall time.

In my opinion, voltage divider can used for frequencies up to 20MHz without compensation capacitor.
Most uses is above 1MHz (i2C, SPI, 1W, etc), that voltage divider works very well.
Also, active shifter can be improved if take in consideration input capacitance, and others characteristics of input and output pins of devices.
I not simulated parasitic inductance of resistor and others parts of circuit.

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