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Amplifier Decoupling: EMI vs MLCC

EMI Amplifier Decoupling

Test #1

  • Uses an AD8221 instrumentation amplifier
  • Pin pattern is amenable to EMI circuit 1” use
  • +V / -V power pins are on the same side of the device
Photo of an X2Y capacitor used with an AD8221 amplifier Schematic diagram of the AD8221 amplifier decoupling circuit

Test #2

  • Uses an INA121 instrumentation amplifier
  • Pin pattern is amenable to EMI”circuit 2” use
  • +V / -V power pins are on the opposite sides of the device
Photo of an X2Y capacitor used with an INA121 amplifier Schematic diagram of the INA121 amplifier decoupling circuit

Compare Bypass Conventional MLCC vs. EMI

Test #1

  • Compares external noise rejection of power bypass networks
    – Single EMI 330nF rated part, versus four total MLCCs
  • Noise voltage measured directly across IC pins
Diagram of the device-under-test configurations for amplifier decoupling

PCB Configuration

Test #1

  • Two layer 1.5mm PCB
  • Single EMI 330nF rated part, versus four total MLCCs
  • Noise voltage probed directly across IC pins at IC body
Diagram of the PCB configuration used for amplifier decoupling tests

PCB Configuration

Test #1

  • Equalized layout parasitics
  • Ground attachment matched between set-ups
  • Capacitor set-backs matched between set-ups
Diagram showing matched ground and capacitor set-back distance

Noise Injection

  • 200ps edges
    – Comparable to memory
  • 100MHz pulse rate
    – Isolate any cavity / capacitor ringing
  • 400mV on 15V power
    – Alternate tests:
    +15V / -15V
  • – 2.7% pp
Chart of noise injection results at positive 15 volts Chart of noise injection results at negative 15 volts

VCC_15V+

Chart of Vcc noise at positive 15 volts
  • EMI 3.7mV pp, conventional 5.6mV pp
  • Conventional noise 151% greater than EMI

VCC_15V-

Chart of Vcc noise at negative 15 volts
  • EMI 5.3mV pp, conventional 8.9mV pp
  • Conventional noise 168% greater than EMI

Compare Bypass Conventional MLCC vs. EMI

Test #2

  • Amplifier power pin pattern amenable to EMI “circuit 2” use
    – the +/- power pins are on the same side of the device
  • Compares single EMI 100nF rated (200nF total) per pin vs. a single MLCC 220nF per pin
Diagram of the second set of device-under-test configurations

PCB Configuration

Test #2

  • Ground attachment is matched between set-ups
  • Capacitor set-backs are matched between set-ups
  • Compares single EMI 100nF rated (200nF total) per pin vs. a single MLCC 220nF per pin
Diagram showing a second matched ground and capacitor set-back distance

VCC_15V-

Chart of Vcc noise measuring 3.6 millivolts at negative 15 volts
  • EMI 3.6mV pp, conventional 10.1mV pp
  • Conventional noise 280% greater than EMI

VCC_15V+

Chart of Vcc noise measuring 4.9 millivolts at negative 15 volts
  • EMI 4.9mV pp, conventional 17.3mV pp
  • Conventional noise 353% greater than EMI

Summary

Test #1

  • Conventional filter using two capacitor values per power pin, four capacitors total results in >= 150% voltage noise compared to just one EMI used for both power pins.

Test #2

  • Conventional filter using one capacitor value per power pin, two capacitors total results in >= 280% voltage noise compared to one EMI used for each power pin.
  • Benefits: smaller space, fewer parts, better economy and performance when using EMI components.

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