EEG Signal Level Shifting

flux

2:18

Checking schematic details.

Following design guidelines.

Analyzing OP5 details.

Examining resistor connections.

Accessing knowledge base.

Analyzing the schematic.

Analyzing OP5 and connections.

Clarifying circuit modifications.

You're close, but the final stage isn’t level-shifting correctly right now.
What I see
  • OP5 is wired as a unity-gain follower (OUT shorted to IN1 on [Net 18](Net 18)), and its input node [Net 20](Net 20) is hard-tied to three things: the AD620 output (IC2), the upper leg of R21, and the lower leg of R20.
  • R20 and R21 are both 500 Ω, forming a very stiff divider between the positive rail ([Net 21](Net 21)) and GND. This divider is loading and effectively pinning the IC2 output instead of just providing a reference.
  • IC2 is tied to GND, so the AD620 output is referenced to 0 V. For single-supply conversion to an MCU ADC, the instrumentation amp’s REF should be biased to mid-rail.
What to change to get a clean 0–5 V ADC signal
  • Provide a 5 V single supply to the final stage and use a rail-to-rail op-amp there (so it can swing close to 0 and 5 V).
  • Create a 2.5 V mid-rail reference using a divider that is not tied directly to the signal:
    • Use the divider (R20/R21) only to generate mid-rail; do not tie it directly to IC2.
    • Buffer that mid-rail with OP5 (configure OP5 as a follower of the divider) to make a low-impedance Vmid.
  • Shift the instrumentation amp output to mid-rail:
    • Drive IC2 from the buffered 2.5 V Vmid (instead of GND). That centers the AD620 output around 2.5 V automatically.
  • Final gain/filter stage to the Arduino:
    • Reconfigure OP5 as a non-inverting amplifier for the AD620 output (now centered at 2.5 V) with the gain you need so peaks stay within 0–5 V.
    • Add a simple anti-alias RC at the ADC input and keep the source impedance low enough for the ADC’s sample-and-hold to settle within the acquisition time.
Why
  • Single-supply op-amp chains need mid-rail biasing; tying the signal to a stiff divider (500 Ω/500 Ω) forces the amplifier to fight the divider. The mid-rail must be generated and buffered separately, then used as a reference (especially at IC2).
  • For ADCs, the bias/reference network that feeds the ADC must be low impedance so the sample-and-hold capacitor fully charges during Tacq.
A couple quick questions to size the gain and input filter correctly:
  1. Do you actually have a 5 V rail available on this board (or from the Arduino) to power the final stage?
  2. Which Arduino/ADC reference are you using (5 V default or an external ref), and what sampling rate?
  3. What amplitude do you currently see at IC2 after the earlier gain stages (approximate peak-to-peak)?
If you want, I can restructure the final stage as above (move the divider off the signal, bias IC2 to mid-rail, and set OP5 up for the correct gain) once you confirm the rail and target amplitude.
Confirm 5V and ADC ref
Ask how to bias REF
Provide signal amplitude
Request op-amp selection
Set divider values
68 days

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-9V
R12
Resistance
27kΩ
C6
Capacitance
120nF
C7
Capacitance
100nF
R11
Resistance
27kΩ
R14
Resistance
330kΩ
TL084CN
R18
Resistance
1.2kΩ
9V
R19
Resistance
120 Ω
R15
Resistance
330kΩ
C10
Capacitance
100nF
R17
Resistance
110kΩ
C5
Capacitance
120nF
C8
Capacitance
100nF
TL084CN
TL084CN
R10
Resistance
3.9kΩ
R13
Resistance
3.9kΩ
R16
Resistance
110kΩ
C9
Capacitance
100nF
R20
Resistance
1kΩ
INA128
  • Ground
    Ground
    A common return path for electric current. Commonly known as ground.
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    Wirelessly connects power nets on schematic. Identical to the net portal, but with a power symbol. Used to organize schematics and separate functional blocks. To wirelessly connect power net portals, give them the same designator. #portal #power
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    A generic fixed resistor ideal for rapid circuit topology development. Its footprint automatically adapts based on the selected package case code—supporting 0402, 0603, 0805, 1203, and many other standard SMD packages, as well as axial horizontal and vertical configurations. Save precious design time by seamlessly add more information to this part (value, footprint, etc.) as it becomes available. Standard resistor values: 1.0 ohm, 10 ohm, 100 ohm, 1.0k ohm, 10k ohm, 100k ohm, 1.0M ohm 1.1 ohm, 11 ohm, 110 ohm, 1.1k ohm, 11k ohm, 110k ohm, 1.1M ohm 1.2 ohm, 12 ohm, 120 ohm, 1.2k ohm, 12k ohm, 120k ohm, 1.2M ohm 1.3 ohm, 13 ohm, 130 ohm, 1.3k ohm, 13k ohm, 130k ohm, 1.3M ohm 1.5 ohm, 15 ohm, 150 ohm, 1.5k ohm, 15k ohm, 150k ohm, 1.5M ohm 1.6 ohm, 16 ohm, 160 ohm, 1.6k ohm, 16k ohm, 160k ohm, 1.6M ohm 1.8 ohm, 18 ohm, 180 ohm, 1.8K ohm, 18k ohm, 180k ohm, 1.8M ohm 2.0 ohm, 20 ohm, 200 ohm, 2.0k ohm, 20k ohm, 200k ohm, 2.0M ohm 2.2 ohm, 22 ohm, 220 ohm, 2.2k ohm, 22k ohm, 220k ohm, 2.2M ohm 2.4 ohm, 24 ohm, 240 ohm, 2.4k ohm, 24k ohm, 240k ohm, 2.4M ohm 2.7 ohm, 27 ohm, 270 ohm, 2.7k ohm, 27k ohm, 270k ohm, 2.7M ohm 3.0 ohm, 30 ohm, 300 ohm, 3.0K ohm, 30K ohm, 300K ohm, 3.0M ohm 3.3 ohm, 33 ohm, 330 ohm, 3.3k ohm, 33k ohm, 330k ohm, 3.3M ohm 3.6 ohm, 36 ohm, 360 ohm, 3.6k ohm, 36k ohm, 360k ohm, 3.6M ohm 3.9 ohm, 39 ohm, 390 ohm, 3.9k ohm, 39k ohm, 390k ohm, 3.9M ohm 4.3 ohm, 43 ohm, 430 ohm, 4.3k ohm, 43K ohm, 430K ohm, 4.3M ohm 4.7 ohm, 47 ohm, 470 ohm, 4.7k ohm, 47k ohm, 470k ohm, 4.7M ohm 5.1 ohm, 51 ohm, 510 ohm, 5.1k ohm, 51k ohm, 510k ohm, 5.1M ohm 5.6 ohm, 56 ohm, 560 ohm, 5.6k ohm, 56k ohm, 560k ohm, 5.6M ohm 6.2 ohm, 62 ohm, 620 ohm, 6.2k ohm, 62K ohm, 620K ohm, 6.2M ohm 6.8 ohm, 68 ohm, 680 ohm, 6.8k ohm, 68k ohm, 680k ohm, 6.8M ohm 7.5 ohm, 75 ohm, 750 ohm, 7.5k ohm, 75k ohm, 750k ohm, 7.5M ohm 8.2 ohm, 82 ohm, 820 ohm, 8.2k ohm, 82k ohm, 820k ohm, 8.2M ohm 9.1 ohm, 91 ohm, 910 ohm, 9.1k ohm, 91k ohm, 910k ohm, 9.1M ohm #generics #CommonPartsLibrary
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    1.5M
  • Generic Capacitor
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    A generic fixed capacitor ideal for rapid circuit topology development. You can choose between polarized and non-polarized types, its symbol and the footprint will automatically adapt based on your selection. Supported options include standard SMD sizes for ceramic capacitors (e.g., 0402, 0603, 0805), SMD sizes for aluminum electrolytic capacitors, and through-hole footprints for polarized capacitors. Save precious design time by seamlessly add more information to this part (value, footprint, etc.) as it becomes available. Standard capacitor values: 1.0pF, 10pF, 100pF, 1000pF, 0.01uF, 0.1uF, 1.0uF, 10uF, 100uF, 1000uF, 10000uF 1.1pF, 11pF, 110pF, 1100pF 1.2pF, 12pF, 120pF, 1200pF 1.3pF, 13pF, 130pF, 1300pF 1.5pF, 15pF, 150pF, 1500pF, 0.015uF, 0.15uF, 1.5uF, 15uF, 150uF, 1500uF 1.6pF, 16pF, 160pF, 1600pF 1.8pF, 18pF, 180pF, 1800pF 2.0pF, 20pF, 200pF, 2000pF 2.2pF, 22pF, 220pF, 2200pF, 0.022uF, 0.22uF, 2.2uF, 22uF, 220uF, 2200uF 2.4pF, 24pF, 240pF, 2400pF 2.7pF, 27pF, 270pF, 2700pF 3.0pF, 30pF, 300pF, 3000pF 3.3pF, 33pF, 330pF, 3300pF, 0.033uF, 0.33uF, 3.3uF, 33uF, 330uF, 3300uF 3.6pF, 36pF, 360pF, 3600pF 3.9pF, 39pF, 390pF, 3900pF 4.3pF, 43pF, 430pF, 4300pF 4.7pF, 47pF, 470pF, 4700pF, 0.047uF, 0.47uF, 4.7uF, 47uF, 470uF, 4700uF 5.1pF, 51pF, 510pF, 5100pF 5.6pF, 56pF, 560pF, 5600pF 6.2pF, 62pF, 620pF, 6200pF 6.8pF, 68pF, 680pF, 6800pF, 0.068uF, 0.68uF, 6.8uF, 68uF, 680uF, 6800uF 7.5pF, 75pF, 750pF, 7500pF 8.2pF, 82pF, 820pF, 8200pF 9.1pF, 91pF, 910pF, 9100pF #generics #CommonPartsLibrary
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  • 875105359001
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