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Hi !

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Q1)

Our customer are planing to make evaluation board to change "single-CVBS" to "Diff-CVBS" to input ADV7281A-M.

To connect to ADI video decoder,  I think AC-coupling is necessary.

At ADA4433-1 datasheet page.22 Figure 55, there is "Difference Amplifier in a DC-Coupled Configuration" reference circuit.

Do you have "Difference Amplifier in a AC-Coupled Configuration" reference circuit?

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Q2)

Our customer will use like this.

Do our customer have to do AC-Coupling between "CVBS source" and "ADA4433-1"?

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Q3)

Do our customer have to do AC-Coupling between "ADA4433-1 source" and "ADV7281A-M"?

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Q4)

At ADA4433-1 datasheet Page.15, this is written.

Does this mean whether AC-coupled or DC-coupled, we do not change the connection of -IN and we need to bias with half the amplitude of + IN?

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Q5)

At ADA4433-1 datasheet Page.15, this is written.

Our customer are planing to use these value for each components.

C1 = 22uF

R1 = 62kOhm

R2 = 27kOhm (1V bias

*Fc = 0.26Hz.

How should they decide these value for each components?

Do you think that there is no problem with these values?

How should the cutoff frequency of the high-pass filter be taken into consideration while considering the NTSC signal frequency? (Does sag do not occur?)
Since 1Vpp signal is supposed to be input, is it OK to understand that 1.0 V bias is OK?
When deciding each constant, is there a point to consider other than the cutoff frequency and the bias voltage?

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Best ragards

Kawa

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• Hi Kawa,

Q1) What do you mean equal? Equal to something?

What I meant is that the offset levels on each input pins +IN and -IN should be on an equal dc level.

Q2)Do you mean that the output offset will be equal to input bias voltage so we have to understand that maybe output offset level will drift little because the bias level is made by divide resister and the resister value will change by temperature?

In this question you can refer to the figures below.

The output common mode voltage (Vocm) at the output is set at the midpoint supply which is 1.65V. As seen if the negative input is biased half of the +IN voltage, the voltage will now then extend to only 2.15V and 1.15V which is more efficient than the first.

You are correct for the resistor divider, to avoid such drift please use a precise and low thermal coefficient resistor.

Q3)Which components are too large? C1?

The resistors for biasing, such values will introduce loading effects on the input impedance of the amplifier.

Q4)How do our customer decide resistor value of biasing which should be equal for proper matching?

Q5)I suppose that AC coupling is necessary for ADA4433-1. Can you tell me the recommended value of each value in the attached circuit diagram? Should our customer make like this?(R1 = R3, R2 = R4)

Yes the resistor values should be R1 = R3, R2 = R4.

Actually, if your CVBS source is what have you said is 1Vpp, and do not vary AC coupling is not necessary. You can just put a dc bias circuit the same on the Figure 53 on the datasheet.

But if the CVBS source varies in magnitude AC coupling is necessary and we have to compute for the resistor values for biasing and the value of the capacitor for the proper bandwidth.

I'm currently contacting the product expert for the proper consideration for input loading and bandwidth. I'll get back to you tomorrow.

Regards,
Goz

• Hi Kawa,

Q1) What do you mean equal? Equal to something?

What I meant is that the offset levels on each input pins +IN and -IN should be on an equal dc level.

Q2)Do you mean that the output offset will be equal to input bias voltage so we have to understand that maybe output offset level will drift little because the bias level is made by divide resister and the resister value will change by temperature?

In this question you can refer to the figures below.

The output common mode voltage (Vocm) at the output is set at the midpoint supply which is 1.65V. As seen if the negative input is biased half of the +IN voltage, the voltage will now then extend to only 2.15V and 1.15V which is more efficient than the first.

You are correct for the resistor divider, to avoid such drift please use a precise and low thermal coefficient resistor.

Q3)Which components are too large? C1?

The resistors for biasing, such values will introduce loading effects on the input impedance of the amplifier.

Q4)How do our customer decide resistor value of biasing which should be equal for proper matching?

Q5)I suppose that AC coupling is necessary for ADA4433-1. Can you tell me the recommended value of each value in the attached circuit diagram? Should our customer make like this?(R1 = R3, R2 = R4)

Yes the resistor values should be R1 = R3, R2 = R4.

Actually, if your CVBS source is what have you said is 1Vpp, and do not vary AC coupling is not necessary. You can just put a dc bias circuit the same on the Figure 53 on the datasheet.

But if the CVBS source varies in magnitude AC coupling is necessary and we have to compute for the resistor values for biasing and the value of the capacitor for the proper bandwidth.

I'm currently contacting the product expert for the proper consideration for input loading and bandwidth. I'll get back to you tomorrow.

Regards,
Goz

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