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[TEST] LD Driver Current Noise #17

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opened 2022-11-16 13:58:58 +08:00 by topquark12 · 2 comments
topquark12 commented 2022-11-16 13:58:58 +08:00
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linuswck commented 2024-05-06 17:37:34 +08:00
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DUT Specs

300pA/rt Hz current noise @ 1kHz
300nA RMS Noise (10Hz - 1MHz)

Measurement Equipment

Keysight N9020a
Noise Floor: ~9nV/sqrt(Hz) @ 1kHz (DC Coupling) (@100Hz Resolution) (0-10MHz)

Post Amplifier

Description

An AC Coupling Post Amplifier is needed to amplify the noise of DUT to measure the LD Noise Specs. Since we are interested at 10Hz - 1MHz range of noise, an instrumental amplifier is the best option in this application.

Target Specs

AC Coupling
Gain: 2000x on Post Amp on top of the 10Ohm load
Noise Floor: 3 Times better than test equipment (4.5nV/sqrt(Hz))
Frequency Range: 10Hz-1MHz

TI INA848 (2000x Fixed Gain) Instrumental Amplifier
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Simulation Setup

image

Simulated Frequency Response

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Simulated Noise Floor (without Resistor Noise)

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Theoretical Noise Calculation

Resistor Current noise = 0.3k Ohm * 1.85pA/sqrt(Hz) = 0.555nV/sqrt(Hz)
Voltage Noise = 1.5nV/sqrt(Hz)
Resistor Johnson Noise(@25 Degree) = sqrt(4*(1.38×10^-23)*(25+273.5)*0.3k Ohm) =2.22nV/sqrt(Hz)
Theoretical Noise Floor = sqrt(2.22^2 + 1.5^2 + 0.555^2) = 2.739nV/sqrt(Hz) < 3nV/sqrt(Hz)

Reference Materials

Reference Test Setup/Other Laser Current Source for Comparison: https://www.thinksrs.com/downloads/pdfs/applicationnotes/LDC%20Note%205%20Current%20Noise.pdf
Design Consideration: https://www.ti.com/lit/ug/tidu016/tidu016.pdf?ts=1714986284737&ref_url=https%253A%252F%252Fwww.google.com%252F
Reference Circuit Design: https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1276887/ina849-reading-small-ac-current-noise
PCB Design Files: https://git.m-labs.hk/linuswck/INA848_INA849_Gain_Block

# DUT Specs 300pA/rt Hz current noise @ 1kHz 300nA RMS Noise (10Hz - 1MHz) # Measurement Equipment Keysight N9020a Noise Floor: ~9nV/sqrt(Hz) @ 1kHz (DC Coupling) (@100Hz Resolution) (0-10MHz) # Post Amplifier ## Description An AC Coupling Post Amplifier is needed to amplify the noise of DUT to measure the LD Noise Specs. Since we are interested at 10Hz - 1MHz range of noise, an instrumental amplifier is the best option in this application. ## Target Specs AC Coupling Gain: 2000x on Post Amp on top of the 10Ohm load Noise Floor: 3 Times better than test equipment (4.5nV/sqrt(Hz)) Frequency Range: 10Hz-1MHz TI INA848 (2000x Fixed Gain) Instrumental Amplifier ![image](/attachments/fbbe8e47-bccd-4ca5-9bd0-e13fa47a30e2) ## Simulation Setup ![image](/attachments/1ee3ecf4-26ab-42fd-9618-7a1c18c52b73) ### Simulated Frequency Response ![image](/attachments/5a010bec-6249-4245-8381-fc1b36b4f8bd) ### Simulated Noise Floor (without Resistor Noise) ![image](/attachments/9a3ecf32-5114-4177-bee9-3a801d56efad) ## Theoretical Noise Calculation Resistor Current noise = 0.3k Ohm * 1.85pA/sqrt(Hz) = 0.555nV/sqrt(Hz) Voltage Noise = 1.5nV/sqrt(Hz) Resistor Johnson Noise(@25 Degree) = sqrt(4*(1.38×10^-23)*(25+273.5)*0.3k Ohm) =2.22nV/sqrt(Hz) Theoretical Noise Floor = sqrt(2.22^2 + 1.5^2 + 0.555^2) = 2.739nV/sqrt(Hz) < 3nV/sqrt(Hz) # Reference Materials Reference Test Setup/Other Laser Current Source for Comparison: https://www.thinksrs.com/downloads/pdfs/applicationnotes/LDC%20Note%205%20Current%20Noise.pdf Design Consideration: https://www.ti.com/lit/ug/tidu016/tidu016.pdf?ts=1714986284737&ref_url=https%253A%252F%252Fwww.google.com%252F Reference Circuit Design: https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1276887/ina849-reading-small-ac-current-noise PCB Design Files: https://git.m-labs.hk/linuswck/INA848_INA849_Gain_Block
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linuswck commented 2024-05-24 18:24:00 +08:00
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WIP: updates for possible typos

Test Setup

Measurements are taken when all DUT, load resistor, gain block, SA reaches their thermal equilibrium state. Kirdy outputs 300mA(design specs full range) to a VPR220 Series 10Ohm Ultra Precision Power Resistor. Kirdy's TEC is powered off during the test.

Changes Made to the Above Gain Block Circuit

To eliminate possible unwanted DC bias voltage, the output filter of the amplifier is instead being configured to be a RC high pass filter(50Ohm +47uF x 10 Ceramic Caps). Since DC coupling is only specified in the range between 0Hz to 1MHz. DC coupling is used with 50Ohm impedance on the SA input. Therefore, it should gives us a cutoff frequency of 3.3863Hz(@-3dB).

Methodology

Irms(10Hz to 1MHz) Measurement:

Vrms Measurement can be calculated from noise spectral density over a bandwidth. KEYSIGHT provides this equation that they used in the marker noise calculation of the SA.

To calculation vrms value, TI apps notes provides the following equation.

RMS noise = sqrt( sum( (noise density for bandwidth ^2)*bandwidth of the frequency range) )

The app note is doing theoretical calculation of noise but it is applicable in our case.

Measurement is taken in different linear sections due to the limit in FFT points. Generally more point within a range gives much more accurate result. This is especially important for low frequency ranges. Here is the frequency range of linear sections.

  1. 10Hz-100Hz
  2. 100Hz-1kHz
  3. 1kHz-10kHz
  4. 10kHz-100kHz
  5. 100kHz-1MHz

Keysight Signal Analyzer Parameter

Input: DC Coupling 50Ohm Impedance(Required)
Sweep Point: 40001 pts(Maximum)
BW: 1.0 Hz (Minimum)
AVG Type: RMS
AVG Pts: 10
Atten: 6dB(Minimum)
Ext Gain: 66dB (External 2000x Gain)

Noise Density Measurement @1kHz

A noise marker is used for the noise density measurement.

Keysight Signal Analyzer Parameter

Input: DC Coupling 50Ohm Impedance(Required)
Sweep Point: 40001 pts(Maximum)
BW: 1.0Hz
AVG Type: RMS
AVG Pts: 10
Atten: 6dB(Minimum)
Ext Gain: 66dB (External 2000x Gain)

Start Frequency: 0Hz
Stop Frequency: 2kHz
Noise Marker Start Frequency: 950Hz
Noise Marker Stop Frequency: 1050Hz

Measurement Result

image
Overview of the frequency of interests(0Hz-1MHz)

image
Current RMS Noise = 181.4nA < 300nA (Specs)

Noise Density @1kHz = 2.7812nV/rt(Hz)/10Ohm = 278.12 pA/rt(Hz) < 300pA/rt(Hz)
Noise at 1kHz

The design is within the target specs.

WIP: updates for possible typos # Test Setup Measurements are taken when all DUT, load resistor, gain block, SA reaches their thermal equilibrium state. Kirdy outputs 300mA(design specs full range) to a VPR220 Series 10Ohm Ultra Precision Power Resistor. Kirdy's TEC is powered off during the test. # Changes Made to the Above Gain Block Circuit To eliminate possible unwanted DC bias voltage, the output filter of the amplifier is instead being configured to be a RC high pass filter(50Ohm +47uF x 10 Ceramic Caps). Since DC coupling is only specified in the range between 0Hz to 1MHz. DC coupling is used with 50Ohm impedance on the SA input. Therefore, it should gives us a cutoff frequency of 3.3863Hz(@-3dB). # Methodology ## Irms(10Hz to 1MHz) Measurement: Vrms Measurement can be calculated from noise spectral density over a bandwidth. KEYSIGHT provides this [equation](https://docs.keysight.com/kkbopen/how-does-the-noise-marker-function-work-on-my-spectrum-analyzer-577940563.html) that they used in the marker noise calculation of the SA. To calculation vrms value, TI [apps notes](https://www.ti.com/lit/an/snva515c/snva515c.pdf?ts=1716458846903&ref_url=https%253A%252F%252Fwww.google.com%252F) provides the following equation. RMS noise = sqrt( sum( (noise density for bandwidth ^2)*bandwidth of the frequency range) ) The app note is doing theoretical calculation of noise but it is applicable in our case. Measurement is taken in different linear sections due to the limit in FFT points. Generally more point within a range gives much more accurate result. This is especially important for low frequency ranges. Here is the frequency range of linear sections. 1. 10Hz-100Hz 2. 100Hz-1kHz 3. 1kHz-10kHz 4. 10kHz-100kHz 5. 100kHz-1MHz ### Keysight Signal Analyzer Parameter Input: DC Coupling 50Ohm Impedance(Required) Sweep Point: 40001 pts(Maximum) BW: 1.0 Hz (Minimum) AVG Type: RMS AVG Pts: 10 Atten: 6dB(Minimum) Ext Gain: 66dB (External 2000x Gain) ## Noise Density Measurement @1kHz A noise marker is used for the noise density measurement. ### Keysight Signal Analyzer Parameter Input: DC Coupling 50Ohm Impedance(Required) Sweep Point: 40001 pts(Maximum) BW: 1.0Hz AVG Type: RMS AVG Pts: 10 Atten: 6dB(Minimum) Ext Gain: 66dB (External 2000x Gain) Start Frequency: 0Hz Stop Frequency: 2kHz Noise Marker Start Frequency: 950Hz Noise Marker Stop Frequency: 1050Hz ## Measurement Result ![image](/attachments/adbbf0b0-cfd4-4e2f-8e5d-edd9cc871afe) Overview of the frequency of interests(0Hz-1MHz) ![image](/attachments/35975dcf-ace5-4362-ace1-77e72de290db) Current RMS Noise = 181.4nA < 300nA (Specs) Noise Density @1kHz = 2.7812nV/rt(Hz)/10Ohm = 278.12 pA/rt(Hz) < 300pA/rt(Hz) ![Noise at 1kHz](/attachments/5d334bd3-07d4-4734-8585-d6c36b4a93d1) The design is within the target specs.
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