-190 dBV2/Hz Preamplifier for Low Frequency
Noise Measurements
Saburo Yokokura1, Nobuhisa Tanuma1, Munecazu Tacano1,
Sumihisa Hashiguchi2, Josef Sikula3, Yoko Kajiwara4, Masami Hirasita5
1
3
Meisei University, Tokyo, 191-8506, Japan, 2Yamanashi University, Kofu, Japan,
Brno University of Technology, Brno, Czech Republic, 4Bunkyo University, Saitama,
Japan, 5Kinjyou University, Kanazawa, Japan
E-mail:yokokura@ee.meisei-u.ac.jp
Abstract. Low cost highly sensitive preamplifier was made by using the commercially available
operational amplifier AD797 which has the input noise equivalent power of -190 dBV2/Hz with
80 dB amplification.
Keywords: Noise Measurements, Low Noise Preamplifier
PACS: 84.30.Le
INTRODUCTION
The AD797 of Analog Devices Co. has the low input noise of 0.9 nV/(Hz)1/2 and
low total harmonic distortion of -120 dB at audio frequencies. In order to improve the
input sensitivity of the preamplifier, we used 16 AD797 in parallel to the feed back
resistance of 10/100 Ω for each input stage followed by the same single AD797 2nd
stage amplifier. Each stage amplifies 40 dB, and the input noise power must be
reduced to 1/4 for 16 parallel amplifiers, 0.23 nV/(Hz)1/2 (-193 dBV2/Hz). Each device
requires DC current of 30 mA at ±12 V, 12W in total, and fairly large heat sink is
necessary to reduce the heating up of the device. The highest sensitivity of about
-190 dBV2/Hz is realized between 100 Hz and 10 kHz. The input impedance of this
amplifier is 1.5 kΩ with the capacitance of 320 pF.
HIGHLY SENSITIVE PREAMPLIFIER
The preamplifier is the key device in noise measurements and the highly sensitive
wide range reliable preamplifier is necessitated to improve the measuring technology.
Typical preamplifier #113 of PAR/EG&G has the input sensitivity of 1nV/(Hz)1/2
below 100 Hz [1], but we may need further sensitivity to detect more small noise
levels in a wider frequency range. The extension to the lower frequency limit is
required especially in measuring small level RTS noises, and to the higher frequency
limit in assigning the shallow trap levels at elevated temperatures.
CP780, Noise and Fluctuations: 18th International Conference on Noise and Fluctuations-ICNF 2005,
edited by T. González, J. Mateos, and D. Pardo
© 2005 American Institute of Physics 0-7354-0267-1/05/$22.50
693
We used parallel connection of 16 AD797 operational amplifiers to reduce the input
equivalent noise level [2], [3], followed by a single summing amplifier together with
the zero balance circuit and 80 dB gain, as is shown in Fig. 1. The amplifier AD797
made by Analog Devices has the sensitivity of 0.9 nV/(Hz)1/2 with the low total
harmonic distortion of -120 dB at audio frequency. Low feedback resistances (10 Ω,
1 kΩ) were used in the non-inverting amplifier circuit to reduce the input noise and
obtain 40 dB gain. Input noise equivalent power is expected to decrease to 1/161/2,
0.25 nV/(Hz)1/2.
Noninverting amplifier circuits
Input
Summing amplifier circuit
16 circuits
parallel
connection
1.1 mF
Output
15 k
Zero correction circuit
FIGURE 1. Preamplifier Block Diagram.
The coupling capacitor in the input terminal is set 1.1 mF and the resistance 15 kΩ.
The first stage output voltage includes DC voltages induced by the offset voltages
and drifts [4], as well as by the leakage currents within the coupling capacitors and the
circuit boards. These DC voltages are eliminated by the zero correction feedback
circuit in the second stage. In order to reduce the thermal fluctuations appearing in the
low frequency region below 1 Hz, 16 operational amplifiers are enclosed in the 8-Lead
Standard Small Outline Package (SOIC) and further effectively cooled down by the
heat sink via a gel sheet, as shown in Fig. 2. The amplifiers are powered using
batteries at ±6V. Figure 3 shows the printed circuit board of the amplifier.
Heat sink
Gel sheet
Integrated circuit
Printed circuit board
FIGURE 2. Attachment of AD797 to heat sink.
694
FIGURE 3. Printed circuit board of highly sensitive preamplifier.
RESULTS
The noise power spectrum density of our preamplifier was measured by a dynamic
signal analyzer HP35665A. The output noise power spectrum densities are shown in
Fig. 4 for 1. shorted input, 2. input resistance 10 Ω, 3. 10 Ω with coupling capacitor
1.1 mF, 4. 10 Ω with 11mF. The noise equivalent power of the shorted input indicates
-190 dBV2/Hz above 100 Hz, while the equivalent input resistance changes by
frequency between 10 Ω and 1.5 kΩ in the presence of the coupling capacitors
-100
Sv, Nise Power (dBV 2/Hz)
-120
Ιnput resistance 15 kΩ
-140
10 Ω input
10 Ω input, coupling capacitor: 11mF
-160
10 Ω input, coupling capacitor: 1.1mF
-180
Input short
-200
0.1
1
10
100
1000
10000
Frequency (Hz)
FIGURE 4. Noise power spectrum density of preamplifier
695
100000
The resistor thermal noises are shown for various resistors in Fig. 5. The thermal
noise for 10 Ω resistor, −188 dBV2/Hz, is clearly discriminated from the shorted input
noise. In the low frequencies below 10 Hz, the noise increased by the increase of
offset voltage for higher input resistance. The maximum input voltage to the
preamplifier was 100 µV rms.
-100
2
Sv, Nise Ppwer (dBV /Hz)
-120
-140
1 kΩ input
-160
100 Ω input
10 Ω input
-180
Input short
-200
0.1
1
10
100
1000
10000
100000
Frequency (Hz)
FIGURE 5. Noise power spectrum density of resistance noise (10 Ω, 100 Ω, 1 kΩ)
CONCLUSION
The most sensitive preamplifier ever reported was made by AD797. This
preamplifier has the noise equivalent power of -190 dBV2/Hz, and the thermal noise of
10 Ω resistor is apparently discriminated from the background. Further extension to
the low frequency below 10 Hz will definitely improve the noise measurement
technology.
REFERENCES
1. Scofield, J.H., Rev. Sci. Instrum. 58, 985 (1987)
2. Yokokura, S., Tanizaki, H., Tanuma, N., and Tacano, M., “Improved instrumentation for low
frequency noise measurements”, Proc. of ICNF 2003, edited by J. Sikula, CNRL, Prague, 2003, pp.
847-850
3. Sikula, J., Tacano M., Yokokura, S., and Hashiguchi, S., “New tools for fast and sensitive noise
measurements”, Proc. of NATO ARW “Advanced Experimental Methods for Noise Research in
Nanoscale Electronic Devices”, edited by J. Sikula and M. Levinshtein, Kluwer, Dordrecht, 2003,
pp. 345-354
4. Hashiguchi, S., Takemoto, Y., Ohki, M., Tacano, M., and Sikula, J., “Suppression of offset and drift
in a DC amplifier by combination of multiple amplifiers”, this conference proceedings
696