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Low-noise measurement amplifier.



MA01 is a low-noise measurement amplifier for noise measurements on low-impedance sources like power-supplies.
It has a voltage gain of 60 dB and 75 nV noise throughout the audio frequency range.


MA01A schematic.
Fig.1: MA01A schematic.

C10 is input HF decoupling.
C11, C12 are input DC-blocking.
R10 in parallel with RB, R13 and R14 sets the input resistance ( 10 kΩ ).
RB discharges C11, C12 if the input is left floating.
D10..D15 is input over-voltage protection allowing an input current of up to 200 mA.
RA limits the input current and allows the input to be connected directly to a "hot" power supply.
The circuit around U11 provides DC bias for U1A..U1D. An AD797 has an input bias current of up to 3 µA. That is an input bias voltage of 120 mV with a 10 kΩ source resistor and will drive the AD797 outputs into saturation.
U1A..U1D are non-inverting amplifiers with almost 60 dB of gain.
R1A..R1D prevents the AD797s from oscillating.
C4, C5 are DC blocking capacitors.
R4 are summing resistors.
R4A..R4D and R16, R17 provides the remaining gain up to 60 dB.
D91, D92 are reverse voltage protection for the power supply.

Power supply.

The circuit requires a low-noise power supply in the range ±5 V to ±15 V for optimum performance.
I have used the circuit with ±10 V. The PS25ABA was designed for this.

Component selection.


All electrolytic capacitors are general-purpose types with sufficient voltage rating.
C10 and C13 are film capacitors.
All other capacitors are ceramic types.


All unmarked diodes are 1N4148. These are sensitive to ambient light so the circuit must be shielded from AC-powered light.
D91, D92 are 1 A rectifier diodes with sufficient voltage rating. I specify 1N4007 as I use these everywhere (and they were cheaper than 1N4001 the last time I bought).


All resistors are 0.25 W..0.6 W 1% metal-film types.
RB is mounted on the PCB bottom side across C10.
RA is mounted on the switch.


I have only tested the circuit with those shown on the schematic. If you want to try other types, you are on your own.


Specification for the MA01AAA prototype.
Measurements are made on the work-bench without enclosure.
The power-supply is regulated by a PS25ABA low-noise regulator.
All measurements are made with a 100 kΩ load on the MA01 output. The source resistance is 1 Ω unless noted.
The gain as assembled was 0.48 dB too low due to component tolerances. This was adjusted by changing R16 to 2.10 kΩ and R17 to 0 Ω.
The unit dBu is dB referred to 0.775 V.
More detailed measurements are in the file MA01A_Measurements.ods (part of the design files download).

Table 1: Specification for MA01AAA.
Supply current into PS25ABA, positive73 mA
Supply current into PS25ABA, negative71 mA
Voltage gain, 1 kHz60.0 dB
Linearity, 20 Hz - 20 kHz ref. 1 kHz±0.05 dB
Linearity, 10 Hz - 100 kHz ref. 1 kHz+0.1 / -0.6 dB
THD+N, 1 kHz, 80 kHz BW, 17 dBu output level0.006%
THD+N, 20 Hz - 20 kHz, 80 kHz BW, 17 dBu output level<0.015%
Output noise voltage, 22 Hz - 22 kHz-80.2 dBu
Output noise voltage, 400 Hz - 22 kHz (1)-80.4 dBu
Output noise voltage, 10 Hz - 80 kHz-74.1 dBu
Output noise voltage, input shorted, 22 Hz - 22 kHz-80.5 dBu
Output noise voltage, input shorted, 400 Hz - 22 kHz (1)-80.7 dBu
Output noise voltage, input shorted, 10 Hz - 80 kHz-73.8 dBu
Equivalent input noise, 22 Hz - 22 kHz-140.2 dBu ( 75 nV )
Equivalent input noise, input shorted, 22 Hz - 22 kHz-140.5 dBu ( 73 nV )
Board size (length / width / height)81.3 mm / 86.4 mm / 21.5 mm

(1)This measurement checks for low-frequency noise. It is <0.1 dB lower than the 22 Hz - 22 kHz value for white noise. If it is much lower than the 22 Hz - 22 kHz noise, it indicates that there is some mains hum present.


MA01A photo.
Fig.2: Photo of mounted board.

Download MA01A design files.

I have boards available for this project. See the PCBs page.

Known Issues / updates.

RB is missing on the PCB. Mount RB on the PCB bottom side across C10.

Copyright and disclaimer.

Copyright Notice.
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Poul Petersen, C/Faya 14, 35120 Arguineguín, Las Palmas, Spain.
E-mail: diy@poulpetersen.dk

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