USER MANUAL
USER MANUAL
ERMSDCV2
DC..1MHz RMS to DC converter, 85dB Logarithmic output
voltmeter & 80dB 10Hz...100kHz low Noise Amplifier
ERMSDCV2_Manual.odt
OnEAudioProjects
contact@oneaudio.net
www.oneaudio.net
Document Revision Date Modifications Author
A 03/2011 Initial manual ON
Contents
1) What is it ? ..................................................................................................................... 2
2) Instrument overview ...................................................................................................... 2
2.1) Functional diagram ....................................................................................................................... 2
2.2) Front panel description ................................................................................................................. 3
(1) Input coupling switch: ...................................................................................................................................................... 3
(2) RMS and logarithmic voltmeter input: ............................................................................................................................. 3
(3) RMS/LOG voltmeter input overload led indicator ............................................................................................................. 3
(4) LNA input overload led indicator ...................................................................................................................................... 3
(5) LNA input ......................................................................................................................................................................... 3
(6) Power ON led indicator .................................................................................................................................................... 3
2.3) Rear panel description .................................................................................................................. 4
(1) LNA output ....................................................................................................................................................................... 4
(2) DC supply input ............................................................................................................................................................... 4
(3) Logarithmic voltmeter output .......................................................................................................................................... 4
(4) RMS voltmeter output ...................................................................................................................................................... 4
3) RMS/DC and LOG voltmeter operation. .......................................................................... 5
4) Low noise amplifier (LNA) operation. ............................................................................. 5
5) Technical specifications .................................................................................................. 6
5.1) True RMS voltmeter ...................................................................................................................... 6
5.2) Logarithmic voltmeter .................................................................................................................. 6
5.3) 80dB Low noise amplifier ............................................................................................................. 6
ERMSDCV2 front and rear panel view
Page 1/7
1) What is it ?
The ERMSDCV2 is a small box that includes some very useful functions for electronics design and
test, especially in audio domain.
It's main function is a precise wide bandwidth RMS voltmeter. The RMS conversion is performed
using state of the art RMS to DC converter IC, the LTC1968 from Linear Technology.
The second function is a high dynamic range logarithmic voltmeter. An analog output allows to
display the measured AC voltage directly in dBV instead of Volts. What is useful when the dynamic signal
level is high. The design use the AD8307 IC from Analog Devices to achieve it.
The third ERMSDCV2 function is a 80dB 10Hz..100kHz bandwidth low noise amplifier.
This amplifier allow to measure signal level down to micro-volt range, it has been intended to perform
voltage regulators (such LM7805/LM317/TL431 and many others) comparative output noise
measurements in audio band. Of course, it can be used in any application needing to extract low signal
level without adding noise. The RTI noise of the amplifier is lower than 500nVrms...
2) Instrument overview
2.1) Functional diagram
The diagrams shows the main features of the ERMSDCV2 design.
Note the RMS and LOG voltmeter share the same high impedance input.
The LNA block is fully separated, it can be used independently of the voltmeter section.
Page 2/7
2.2) Front panel description
(1) Input coupling switch:
The input coupling switch allow to select the type of voltage measurement.
In “AC” mode, only RMS value of the AC part is computed.
In “AC+DC” mode, it's the RMS value of the AC and DC part of the input voltage that is computed.
(2) RMS and logarithmic voltmeter input:
This is the common input of the RMS and Logarithmic voltmeter(BNC female connector).
This input has a 1MΩ//10pF input impedance. The high input impedance avoids loading the voltage
source to perform precise measurements.
(3) RMS/LOG voltmeter input overload led indicator
This red led light on if the voltage of the RMS/LOG input exceed the upper measurement limit of
+/-2Vpeak.
Note for RMS/DC converter, the peak input voltage must be less than +/-3V to perform correct
measurement, and for the LOG converter this limit is +/-2Vpeak. So, when the red LED light is on, the
measurement range of the LOG voltmeter has been reached, but not the limit of the RMS/DC voltmeter.
This input is protected for voltage up to +/-20V peak.
(4) LNA input overload led indicator
This red led light is on if the input voltage of the LNA input exceed +/-0.5 mVpeak.
Above this limit, the high gain the output make the LNA output saturate.
Note that the DC input voltage to the LNA must not exceed +10VDC.
(5) LNA input
This is the input of the low noise high gain amplifier (BNC female connector).
Because of the very high gain of the amplifier, it is strongly recommended to use coaxial link between
LNA input and the source voltage measured.
(6) Power ON led indicator
This green LED light is on when the unit is powered by correct voltage(more than ~+/-7Vdc).
The pin-out of this connector is :
pin 1 + V in (+7 to +12vdc)→
pin 2 Ground (0V)→
pin 3 - Vin (-7 to -12Vdc)→
pin 4 Not connected→
pin 5 Not connected→
Page 3/7
2.3) Rear panel description
(1) LNA output
This is the low noise amplifier analog output(BNC female connector). Because of input AC
coupling, no DC voltage appear at the output. The output sensitivity is 1Vrms for 100µVrms at the input
(in bandpass frequency 10Hz to 100kHz). The maximum output voltage is about 3.5Vrms for 350µV at
input.
(2) DC supply input
The symmetric DC power supply must be connected there.
The input voltage can be between +/-7V to +/-12V. The required minimum current for each rail is about
150mA.
It is recommended to use clean power supply for proper operation of the ERMSDCV2, especially
for the LNA section that is noise sensitive. For this reason, switching mode power supplies (SMPS)
should be avoided.
(3) Logarithmic voltmeter output
This is the logarithmic voltmeter output (BNC female connector).
The logarithmic output is a DC voltage matching directly voltage expressed in dBV.
For 1V at input you will get 0Vdc at output and for 100mV you will get -200mVdc at output.
This DC output voltage is 0Vdc=0dBV for 1V and -200mVdc=-20dBV for 100mV.
The output slope is -10mV/dB, so it is easy to to directly read voltage in log scale.
(4) RMS voltmeter output
This is the RMS to DC voltmeter output (BNC female connector).
The DC output represents the true RMS (Root-Mean-Square) value of the input signal.
The nominal output sensitivity is 1V/V, it means that for a signal of 1Vrms the output is 1Vdc.
Page 4/7
3) RMS/DC and LOG voltmeter operation.
Simply connect a DC voltmeter on the RMS output and the signal to analyse to the BNC input.
Choose the input coupling mode (AC or AC+DC), the DC voltmeter directly displays the RMS level.
The default nominal full scale input of the RMS voltmeter is 2.5V.But as the input stage of the
RMS/LOG voltmeter is designed as an oscilloscope input, using oscilloscope probes can increase the
input voltage range (just as oscilloscope inputs do).
You can use x10 or x100 scope probes can be used to get 10x or 100x input voltage range. This
then gives a 25V or 250V range instead of 2.5V. This is possible because the internal probe resistor
divider and the RMS input impedance (1MΩ) constitute a voltage divider.
The frequency response when probes are used is roughly the same as without probe, but it is necessary
to trim the probe to obtain a good frequency flatness in all bandwidth.
Using probe has also the advantage to get an higher impedance than a direct connexion.
The probes impedance are :
• No probe (direct connexion) 1MΩ→
• x10 oscilloscope probe 10MΩ→
• x100 oscilloscope probe 100MΩ→
Using probe can avoid parasitic impedance disturbance.
4) Low noise amplifier (LNA) operation.
The LNA can be used for many purpose.
The design has been originally built to compare the noise of different linear voltage regulators.
The application note A N83 from Linear Technology describing the test procedure with low noise amplifier
can be read for further information.
The ERMSDCV2 is widely inspired from various schematics published in LT Applications notes.
The LNA can work together with the RMS/LOG voltmeter. The LNA output has to be connected to
the RMS input. The RMS signal can then be measured to a level down to 1µV.
The resulting DC voltage value on the RMS output must be divided by the 10000 to obtain the real LNA
RMS input voltage value. The one on the LOG output must be reduced by 80dB.
I'm sure that it will be an helpful toll in your lab for your DIY electronics & audio exploration !
Page 5/7
5) Technical specifications
All tests performed with +/-9Vdc linear power supply.
Typical value measured on prototype board.
5.1) True RMS voltmeter
Parameter Min Typ Max Unit
Input impedance
1
MΩ
Input capacitance
15
pF
Linearity (250mV to 2.5V range f=1kHz)
0.5
%
Input voltage range (error<5%, No probe)
0.01
0.014
..
..
2.5
3.5
Vrms
Vpeak
Frequency bandwidth (error<5%) “AC+DC” mode.
“AC” mode (no
probe)
DC
0.01
.. 1000
1000
Khz
kHz
DC output settling time (1V step, 10% to 90%)
350
ms
RMS output sensitivity (trimmed)
1.000
V/V
Maximum input voltage limit (no probe)
.. 20
Vpeak
5.2) Logarithmic voltmeter
Parameter Min Typ Max Unit
DC output logarithmic slope
-10
mV/dB
Linearity from 100µV(-80dBV) to 1V(0dBV) f=1kHz
0.5
dB
Noise floor (Input shorted to ground)
(analog front-end with OPA627+AD817)
-85
56
dBV
µVrms
Frequency bandwidth (error<3dB)
0.01 -- 1800
kHz
Error between 0dBV to -80dBV (1V to 100µV)
-0.5 -- +0.5
dB
5.3) 80dB Low noise amplifier
Parameter Min Typ Max Unit
Nominal LNA Gain (f=1kHz, Vin=100µV, adjusted)
10000
80
V/V
dB
AC input impedance (F=1kHz)
100
Ω
Input referred noise (10Hz..100kHz bandwidth)
500
nVrms
Frequency bandwidth (error<3dB)
0.01 -- 100
kHz
Maximum input voltage
(before saturate)
350
495
µVrms
µVpeak
Maximum output voltage
(before saturate)
3.5
4.95
Vrms
Vpeak
Page 6/7
END OF MANUAL
Copyright OnE Audio Projects 2011
www.oneaudio.net
contact@oneaudio.net
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