The problem of choosing magnification

反倒是fdsf

The problem of choosing magnification [Copy link]

Let’s take a look at the pictures first.

The video in the picture (explained in the datasheet later) is a signal collected by C12666MA, which is amplified by ADA4851 and output from OUT2 to the AD converter. But I don't know how many times this signal needs to be amplified before it can work properly. Why is it designed this way? Is R6 a sampling resistor in the picture? How to get its value? In addition, I attach 3 datasheets about this circuit. Please help me solve the problem. Thank you.

maychang

"Is R6 in the figure a sampling resistor? How do I get its value?"

R6 is not a "sampling resistor". R6 and C13 form a first-order low-pass filter circuit.

maychang

“How many times does this signal need to be amplified before it can work properly?”

That depends on how big your vidio signal amplitude is. The amplification factor is determined by your AD chip input amplitude divided by the vidio signal amplitude.

philips_lu

1. See how large the input voltage range of your AD chip needs; 2. How large the output signal amplitude of the Video signal is; 3. Your accuracy requirements (consider the relationship between the peak value and VREF and the redundant space to prevent oversaturation distortion); 4. R6 and C13 form a low-pass filter. Look at the reference values given in the reference materials. Of course, in addition to the function of the filter, R6 also needs to consider that it plays a role in coupling the input and output of the front and rear chips.

反倒是fdsf

philips_lu posted on 2020-2-14 21:28 1. Depends on how large the input voltage range of your AD chip is; 2. How large the output signal amplitude of the Video signal is; 3. Your accuracy requirements (considering the peak value and VR ...

Hello, sir. I understand a little bit after what you said. I have another question, is the cutoff frequency of this RC filter circuit selected according to the bandwidth of the VEDIO signal, or should it be selected after comprehensive consideration of the two factors after VEDIO passes through the amplifier?

反倒是fdsf

philips_lu posted on 2020-2-14 21:28 1. Depends on how large the input voltage range of your AD chip is; 2. How large the output signal amplitude of the Video signal is; 3. Your accuracy requirements (considering the peak value and VR ...

The datasheet says that the capacitance is >= 590pf and the resistance is < 44Ω. We use 15Ω and 2.7nf here. I calculated the cutoff frequency to be 4MHZ. I wonder how this 4Mhz is determined?

philips_lu

On the contrary, fdsf posted on 2020-2-16 15:52 The datasheet gives a capacitance of >=590pf and a resistance of <44Ω. We use 15Ω and 2.7nf here. I calculated the cutoff frequency...

http://www.elecfans.com/d/646403.html

You can refer to this document. The simple understanding is to assume that the input rating is f, calculate the impedance of the capacitor, and use the voltage division principle to find that the voltage across the filter capacitor C is equal to 0.707 times the input voltage, and calculate the frequency f at this time.