AN1560.1 Datasheet, PDF (1/7 Pages) Intersil Corporation – Making Accurate Voltage Noise and Current Noise Measurements

Application Note 1560
Author: Don LaFontaine
Making Accurate Voltage Noise and Current Noise
Measurements on Operational Amplifiers Down to
0.1Hz
Abstract
Making accurate voltage and current noise
measurements on op amps in the nano volt and femto
amp range can be challenging. This problem is often
addressed by two different approaches. Both approaches
concentrate on reducing the noise of the amplifiers used
to measure the Device Under Test (DUT). The 1^st
approach uses conventional cross-correlation techniques
to remove un-correlated noise and a procedure to
remove the correlated noise contributions made by the
amplifiers used to measure the DUT [1]. The 2^nd
approach, and the subject of this Application Note,
consists of designing a test platform with an effective
background noise at least 10dB lower than the DUT.
To obtain a test platform with this level of performance
requires: the removal of environmental electrical
disturbances, the use of batteries for low noise voltage
sources, the use of a Post Amplifier (PA) to raise the DUT
noise above the measurement system’s noise floor,
control software to measure accurate noise data down to
0.1Hz and processing software to eliminate external
noise and generate the DUT’s voltage (e_n) and current
(i_n) noise plots.
This Application Note will discuss the procedures used to
obtain a test platform that is capable of measuring nano
volts and femto amps down to 0.1Hz. The test platform’s
capability is illustrated by measuring the voltage and
current noise of Intersil’s ISL28190 (Bipolar inputs,
1nV/√Hz) operational amplifier and Intersil’s ISL28148
(MOS inputs, 16fA/√Hz) operational amplifier.
Introduction
To measure an accurate internal noise of an Op Amp, for
a data sheet spec, two types of external noise sources
(Environmental and Johnson) must be removed from the
measurement. Environmental noise is any unwanted
signals arriving as either voltage or current, at any of the
amplifiers terminals or surrounding circuitry. It can
appear as spikes, steps, sign waves or random noise.
This noise can come from anywhere: nearby machinery,
power lines, RF transmitters, lab power supplies or lab
computers. The Environmental noise is minimized by
isolating the DUT in a Faraday cage and powering the
DUT with batteries.
The second external noise source is Johnson noise.
Johnson noise is the noise generated by the external
biasing and gain setting resistors of the DUT and test
platform. Johnson noise is subtracted out from the total
noise measurement through processing software so only
the internal noise of the DUT is reported.
This Application Note will:
1. Discuss basic noise equations (external and
internal) and then use these equations to extract
the DUT noise from our test platform’s noise.
2. Discuss the use of a Post Amplifier (PA) to lower our
HP35670A Dynamic Signal Analyzer’s (DSA)
effective noise floor from 20nV/√Hz to 3nV/√Hz.
3. Illustrate the effectiveness of our Faraday cage to
remove environmental noise.
4. Discuss AC coupling of DUT, PA and DSA.
5. Determine the required gain of the DUT to enable
the test platform to measure voltage noise below
3nV/√Hz.
6. Discuss considerations for choosing the optimum
series resistor R_S to measure current noise.
7. Discuss the Test Platform Algorithm.
8. Present conclusions.
Basic Equations For Calculating
Noise
Johnson noise is the only resistive noise source
considered in this controlled lab study. Other resistive
noise sources such as contact noise, shot noise and
parasitics associated with particular types of resistors
could also be contributing noise in an application.
A typical figure of merit for amplifier noise is noise
density. Voltage-noise density is specified in nV/√Hz,
while current-noise density is usually in units of pA/√Hz
[2]. For simplicity, these measurements are referred to
the amplifier inputs; thus removing the need to account
for the amplifiers gain.
External Johnson Noise
At temperatures above absolute zero, all resistances
generate Johnson noise due to the thermal movement of
charge carriers. This noise increases with resistance,
temperature and bandwidth. The voltage and current
noise are given by Equations 1 and 2 respectively
[3, 4, 5].
External Johnson Voltage Noise
V_n = e_n = 4kTBR
(EQ. 1)
External Johnson Current Noise
i_n =
4_----k_----T_----B_--
R
(EQ. 2)
Where:
k is Boltzmann’s constant (1.38 x 10^-23 J/K).
January 19, 2011
AN1560.1
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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