Re: Understanding LC Meter
OK For mearure LCR you need
1. measuring signal source AC
2. measure the Voltage at the DUT
3. measure the Current over the DUT
4. measure shift between Voltage and Current
One of the methods of measurement is called Auto-balancing bridge method
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Zx=Vx / Ix = Rr * (Vx/Vr)
Problem is how simple and precise make in Low point virtual ground with 0V potential.
Perhaps they try
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My quick design building blocks.
1.Signal source I
Impedance 100Ohm, Amplitude +- 1.25V , frequency 100,1k,10k and 100kHz. Optional with DC bias.
Signal source is DDS AD9833 with 18MHz clock , on output is 3th order LPF on 100kHz,
U3 is 8bit digital potentiometer 1kOhm for precise trimming output amplitude on +-1.25V
U1A amplify 0.6V from DDS on +-1.25V, U1B make optional voltage DC voltage offset for output . R7 define output impedance on 100 Ohm.
SQR for 100kHz output signal is 81.5dB, for 10kHz 91.5 dB etc.
4.Current to voltage converter
Current through the DUT is nulled reverse current passing through resistor. In schematic is used 6 resistor 10,100 Ohm, 1k Ohm, 10k,100k and 1M , resistor is selected with CMOS Switch U3 and 10 ohm with relay K1
Voltage (current) behind DUT is measured with precise amplifier U3 AD8610 + low noise buffer U2 , this on selected R make voltage where
Vr = R*Ir
U4 is used for nulling offset voltage
2,3 Measure Vr and Vx
Vx and Vr is identical therefore, we will discuss only Vx.
First stage, first tree amplifier is precise instrumentation amplifier wit A= 1x ,2x ,5x ,10x and 20x.
Second two amplifier is 5th order active LPF, which reduce noise on 5MHz over -83dB (assuming use 12bit ADC 10MSPS). Last amplifier shift voltage level from +-1.25V to 0-2.5V for ADC and drive ADC.
C5 blocking DC voltage if DC bias is used. R2 define minim current over C5 some may be omitted
Then add enough STM32F303 first 2 ADC use for samplig Vx and second two for samplig Vr. Use some digital filter and have simple, precise LCR
What do you think about it?