How to measure capacity versus bias voltage on MLCCs: Page 2 of 4

January 16, 2015 //By Fons Janssen
How to measure capacity versus bias voltage on MLCCs
Fons Janssen, Principal Member of Technical Staff, Maxim Integrated explains how to measure capacity versus bias voltage on MLCCs.
oscillation period depends on the values of R, C, and the upper and lower thresholds V UP and V LO:

Since 5V, V UP, and V LO are constant, then T 1 and T 2 are proportional to RC. (This is often referred to as the RC time constant.)

The threshold of the comparator is a function of V y, R2 , R3, and the forward voltage of D1 (V DIODE):
   

Where V UP is the threshold for V y = 5V, and V LO is the threshold for V y = 0V. With the given values these thresholds yield to approximately 0.55V for V LO, and 1.00V for V UP.

The circuit around Q1 and Q2 converts the cycle time into a proportional voltage. This works as follows. MOSFET Q1 is controlled by the output of U1. During T 1, Q1 is on, clamping the voltage on C3 to GND. During T 2, Q1 is off, allowing the constant current source (Q2, R5, R6, and R7) to linearly charge C3. [1] As T2 is increased, the voltage on C3 becomes higher. Figure 3 shows the voltage on C3 over three cycles.
   

Figure 3. C3 is clamped to GND during T1 and linearly charged during T2.

The average voltage on C3 (V C3) is equal to:
  

Since I, C3, α, and β are all constant, the average voltage on C3 is proportional to T2 and, therefore, also to C1.

Lowpass filter R8/C4 filters the signal while low-offset op amp U2 (MAX9620) buffers the output so that it can be measured with any voltmeter.

Before measurements can be made, this circuit requires a simple calibration. First the DUT is installed in the circuit, and V BIAS is set to 0.78V (the average of V LO and V UP) so the actual average (DC) voltage across the

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