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Question about Vdd/2 reference voltage

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gaom9

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Hi,
I have a basic question about the Vdd/2 reference voltage.
In many design, the Vdd/2 voltage will be used as the common voltage. But how we can get the voltage?
Can we just use the same resistors in series or the same MOSFET in series to divide the Vdd to two parts? Or we should get it in another way.


Thank you.
Best regards.
 

An opamp has a very low input bias current. Some opamps have FET inputs that have no input bias current. So two 100k resistors in series produce Vdd/2 where they join together.

But they couple half the noise from the power supply into the input of the opamp so a capacitor to ground filters the Vdd/2 and a third resistor feeds the opamp if it is a non-inverting type.
 

Thank you for reply.
But I think two 100K resistors in IC layout is too large.
Besides this method, any other way can do that?
The most important is to prevent the noise from Vdd and less layout area.

Buffered bandgap reference? OK, I will read something about it.
Thank you very much.

Best regards!
 

100k bias resistors are not high.
The maximum input bias current for an ordinary old opamp is 500nA. Therefore it causes an output voltage change of only 25mV if it is not cancelled with the resistor feeding the other input.

The input bias current for a modern FET input opamp is nothing so the two 100k resistors will not cause an output voltage change.

The two 100k resistors are an input impedance of 50k. The source is probably an output impedance of 5k or less. It won't pickup radiated noise..
 

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    gaom9

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Thank you very much, Audioguru.
This problem has puzzle me a long time.
I want to add a Vdd/2 reference voltage to the input signal which is an ac signal to make it in the input range of the OPAMP.
Now I know what should I do.

Best regards.
 

I attached my schematic of AC amplifiers with a non-inverting input and an inverting input biased at Vdd/2.
They are capacitor coupled at their input and output for AC.
The Vdd/2 has a capacitor for filtering.
 

It is very detailed.
I will try it.
And can we make it by adding even MOSFETs with the same size and type between Vdd and Gnd, than in the middle point, the voltage will be Vdd/2.
And it will be less layout area and control the power consumption.

Thank you.
Best regards.
 

I don't know how Mosfets can replace two or three resistors.
Mosfets have a wide range of gate voltage and drain current. Resistors are accurate and use low current.
 

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