Input Offset Voltage

CIRCUIT

Schematic: Op_Amp_voff.asc
Op Amp Symbol: Opamp_2.asy
Op Amp Shematic: Opamp_2.asc
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SPEC IT

Intro	All op amp's have a small input error voltage at their input. How do we know? Ideally, if you shorted together the inputs (0V input), you'd expect to the same at Vo (0V output). However, you'd discover the output is likely slammed to either the high or low power rails. Why? Real-world op amps have a number of internal device mismatches from each input to the output. This causes an effective voltage unbalance at the input. The unbalance is typically modeled as a small error voltage at the positive input.
Definition	voff  - the differential voltage applied at input required to make the output 0V.
Design Goal	Amplifier Offset Error < 5mV due to voff
Design Spec	Vo < 5mV with Vs = 0V and ibias = 0A

DESIGN IT

• Circuit Design
  •  Kcl = (R2+R1)/R1 = 5V/V
  • Choose: R1=10k
  • Calc: R2=40k
• Op Model Param
  • Aol=100000 fu=10e6 slew=10 Ro=1
  • voff = 2mV modeled as voltage source outside the op amp.
    Why? An external Voff help us visulaize and analyze how it effects the circuit.
• Circuit Test
  • Input: no input source (0V)
  • Output:  Vo = 0V
  • Expected Vo < 5mV

TEST IT

• Run a Transient simulation. (.TRAN)
• Plot input error v(vp) and outpuit v(vo)
• Did it meet spec?
  • If vo <= 5mV, then PASS, else FAIL
•  DESIGN ISSUE: The output offset fails to meet spec! 

SOLVE IT

• Incrementally decrease voff.
• Rerun the SPICE sim.
• What's the maximum voff you would accept for Vo < 5mV?

THEORY REFRESH

• How much output error is caused by voff?
• Turns out, voff looks like any other input at the positive terminal. So we can use the familiar output formula from the non-inverting amplifier, vo = voff*Knon.

• Note there's no way around it, the larger your gain, the larger the output error due to voff.
• What happened to our original design?
  • vo = voff * Knon
        = voff * (R1+R2)/R1
        = 2mV*(10k+40k)/10k = 10mV

• Let's solve it
  • What voff max is allowed for a Vo < 5mV?
  • Solve vo = voff*Knon for voff.
  • Then calculate your voff maximum.
• Retest the circuit with the new voff. Did your circuit meet spec?

Select an op amp that flies under the max allowed voff!

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