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Precision
Hub

When GND Isn’t GND, Single-Ended
Circuits Become Differential

Precision
Hub, , 7/3/2014

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The system ground return, or GND, symbol
is often taken for granted when drawing schematics. GND symbols are
placed all over the schematic with the assumption that the different
GNDs will all be at the same electrical potential on the printed
circuit board (PCB). In reality, current flow through the GND
impedance can create voltage differences between the GND connections
on the PCB. Single-ended dc circuits are particularly sensitive to
these GND voltage differences because the expected single-ended
circuit is converted to a differential circuit causing output
errors.

Let’s use the standard non-inverting amplifier circuit shown
below as an example. When the GND potentials of the input source,
VIN, and the input resistor, RI, are equal, the familiar
circuit gain of 1+RF/RI applies. Therefore,
the 100mV input signal is multiplied by a gain of 10 V/V and the
output is 1V.

In the circuit shown in the figure below, a voltage source,
VGND2, has been inserted between the input source GND and
the RI GND connection. The result is a modified transfer
function with the addition of the VGND2 voltage
multiplied by the inverting circuit gain of -
RF/RI. A 10mV difference in GND potentials
reduces the desired 1V output by 90mV to 0.91V. This corresponds to
a 9% relative error compared to the desired 1V output.

In the circuit shown below the circuit transfer function is
further affected when the output voltage is referenced to a third
GND potential, VGND3. The VGND3 voltage will directly
subtract from the previous output transfer function. Therefore, a
20mV VGND3 voltage further reduces the output voltage to
890mV, corresponding to an 11% error compared to the desired 1V
output.

The issues shown in the two examples above can be reduced by
using proper PCB layout techniques to keep GND potentials of the
circuit input source, input resistor, and output voltage at the same
potential. The best solution is to place critical GND connections
physically close to each other using a common “star” GND method.
This will reduce the PCB impedance developed between the GND
connections and will therefore reduce any voltage potential
differences between them. In the example circuit schematic and
layout shown below, the distances between the GND connection of the
input source, output voltage, and input resistor are all located
close together on the top layer of the PCB. This should prevent the
single-ended circuit from becoming a differential one!

In summary, next time you’re having issues with dc circuit
performance verify that the voltage potentials of all important GND
connections are equal.

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Heinzelmann

7/21/2014 3:43:33 AM
User Rank Newbie

Re: ATE

I think the board layout could be
improved.

Especially the currend of C1 must circumvent the
input terminal to a large amount.

The ground pads of C1 and
C2 should be as close as possible to the gnd  of the output
terminal.

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Victor
Lorenzo
7/15/2014 7:17:14 AM
User Rank Blogger

Re: Ground filling

@DaeJ, ground noise can come from
many sources and several of them may overlap. Fortunately it is not
always completely random and gives us a chance to deal with
it.

In many cases, high frequency and high current signals
return paths contribute as ground noise which interferes to low
level signal paths.

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Victor
Lorenzo
7/15/2014 7:11:13 AM
User Rank Blogger

Re: Ground filling

@DaeJ, FYI, band-reject and
all-pass filters are two more filter types which are used for
removing a unwanted frequency band and for compensating for phase
shifts, respectively.

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DaeJ

7/13/2014 8:34:48 AM
User Rank Master

Re: Ground filling

What kind of filtering would be
applicable for ground issue? There are three filtering methods: high
pass; low pass; and band pass. Plus, I think that ground signal
would be a random signal.

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vasanjk

7/12/2014 8:57:33 PM
User Rank Newbie

Ground filling

PrecisionHub

I think
ground filling is done by designers without much thought. One of my
friends argues that by filling, the ground differentials could be
done away with. Can this be applicable to all
situations?

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DaeJ

7/10/2014 1:01:28 PM
User Rank Master

Re: ATE

Let us say that board might have
high voltage transformer which converts AC to DC and there is a few
signal conditional circuit less than 5[v]. In this case, incorrect
design causes the noise as well as a highly potential ground
voltage. Plus, if this kind of board is integrated into high speed
communication module, it becomes more complex board and capturing
the precise signal would be a challenge, unless engineer follows the
correct process.

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etnapowers

7/10/2014 11:19:09 AM
User Rank Newbie

Re: Confusion between noise and
ground issue

@fasmicro: Really nice link thank
you for this. The tutorial of Texas Instruments are a great resource
for an engineer who has to design and debug a circuit board, there
are good suggestions to improve the effectiveness of the testing
process and the reliability of the overall
system.

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etnapowers

7/10/2014 10:40:25 AM
User Rank Newbie

Re: Confusion between noise and
ground issue

A simulator of the effect of PCB
load for the circuit is highly recommended to optimize the
effectiveness of the design process.

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etnapowers

7/10/2014 10:35:50 AM
User Rank Newbie

Re: Confusion between noise and
ground issue

@goafrit2: I agree with you and I
add that the signal paths have to be separated in the PCB layout ,
to avoid cross-talking.

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etnapowers

7/10/2014 8:45:41 AM
User Rank Newbie

Re: ATE

"it might be suggested that
engineer designs the circuit better for high power distribution with
AC."

@DaeJ: could you specify what do you mean
by high power distribution with AC?

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