The video signal carrying capacity close to 1MHz is not enough. What op amp can I use to design a voltage follower?

XuYong虚庸

The video signal carrying capacity close to 1MHz is not enough. What op amp can I use to design a voltage follower? [Copy link]

 

As the title says, the video signal current output by the linear array CMOS is too small, and the input resistance of the AD module used is only 50ohm. I want to use an op amp to design a voltage follower. What op amp is more suitable?

maychang

There are many models of op amps with unity gain bandwidth of tens of MHz.

btty038

AD822 LM2904 ...........

chunyang

There are many models. You can go to the official websites of TI, ADI, LT, Shengbang, etc. to choose the model.

XuYong虚庸

maychang posted on 2020-12-9 14:56 There are many models of op amps with unity gain bandwidth of tens of MHz.

I also need a higher slew rate. Do you have any recommendations? Thanks for your answer

maychang

XuYong Xuyong posted on 2020-12-9 16:12 I also need a higher slew rate. Do you have any recommendations? Thank you for your answer

“I also need a higher slew rate here.”

How high is “relatively high”? Give a numerical value.

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maychang posted on 2020-12-9 16:32 "I also need a higher slew rate" How high is "higher"? Give a value.

5V/us at least

maychang

XuYong posted on 2020-12-9 16:34 5V/us at least

If 5V/us is required, then the very old NE5532 can do the job. The gain bandwidth product is 10MHz and the slew rate is 8V/us.

XuYong虚庸

maychang posted on 2020-12-9 17:30 If 5V/us is required, then the very old NE5532 can do it. Gain bandwidth product 10MHz, slew rate 8V/us.

OK, thank you. I have another question. When the op amp is used as a voltage follower, do I need to consider the maximum output current parameter of the op amp? The current of my current signal is too small, and the AD cannot collect it.

maychang

XuYong Xuyong posted on 2020-12-9 19:07 OK, thank you. Another question, when the op amp is used as a voltage follower, do I need to consider the maximum output current parameter of the op amp? My current signal is...

Your AD input resistance is 50 ohms. How much voltage or current do you need?

If the op amp output current is not large enough, it is also easy to expand the current using discrete transistors.

Of course, it might be better to choose a new op amp with sufficient output current.

XuYong虚庸

maychang posted on 2020-12-9 19:37 Your AD input resistance is 50 ohms, how much voltage or current do you need? If the output current of the op amp is not large enough, it is also very simple to expand the current with discrete transistors...

Taught, thank you.

S3S4S5S6

XuYong posted on 2020-12-9 16:34 5V/us at least

"I also need a higher slew rate here." I was terrified when I heard this, thinking it would cost at least thousands of

The result is 5V/us

XuYong虚庸

S3S4S5S6 posted on 2020-12-17 10:05 "I also need a higher slew rate here." I was terrified when I heard this, thinking that at least thousands of knots would be needed...

Hello, I used this circuit to simulate, and the current output by the op amp is still too small. Can you help me design a push-pull circuit to increase the driving capability of the op amp?

S3S4S5S6

XuYong Xuyong posted on 2020-12-18 16:27 S3S4S5S6 posted on 2020-12-17 10:05 "I also need a higher slew rate here." When I heard this, I was...

"The video signal current output by the linear array CMOS is too small. The input resistance of the AD module used is only 50ohm. I want to use an op amp to design a voltage follower. What op amp is more suitable?" This is the problem you mentioned at the beginning. This problem has little to do with the impedance of the ADC. Your question can be broken down as follows:

1. The video signal current output by the linear array CMOS is too small

2. I want to use an op amp to design a voltage follower (the purpose is to condition the signal in 1)

3. ADC sampling

First, the video signal current output by the linear array CMOS is too small. What does "too small" mean? Is it mA, nA, pA, or fA?

Since the current is too small, we need to make an I/V conversion circuit and use a transimpedance amplifier to convert the current into a voltage signal.

The converted voltage signal is then followed. This level of follower is usually used for isolation, because the input impedance of the op amp is very large and will not affect the previous I/V conversion circuit, while the output impedance is small and the load capacity is not bad.

The selection of ADC is the key. You need to understand the relationship between the resolution of ADC and the minimum change of your signal. There is no doubt that the resolution of ADC should be greater than the minimum change of the signal, otherwise the change of your signal will be submerged in the resolution of ADC.

S3S4S5S6

XuYong Xuyong posted on 2020-12-18 16:27 S3S4S5S6 posted on 2020-12-17 10:05 "I also need a higher slew rate here." When I heard this, I was...

This is the I/V conversion circuit recommended in the ADI chip manual. The relationship between the output V and the input I is shown in the red box expression. RF usually selects a resistor with a megohm accuracy of at least 1%. The input current can be at the nA level, and then select the ADC buff

Of course, you can adjust the relevant parameters appropriately to make a follower, or you can perform appropriate secondary amplification on the input of the I/V conversion and then input it into the ADC. In addition, the ADC does not directly detect current, but usually converts it into voltage. The original poster emphasized that the output current is not enough for the ADC to recognize. Generally speaking, it should be that the selection of the ADC is unreasonable, that is, the number of bits of the ADC is too low, resulting in insufficient resolution. Therefore, there is no way but to require the performance of the buffer BUFF to be excellent (here it shows that an op amp with a large output current, that is, a strong driving capability, is needed) to offset the shortcomings of the ADC. Usually, high-speed, high-resolution ADCs are also very expensive, so appropriately improving the performance of the BUFF is also a way to reduce costs.

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S3S4S5S6 Published on 2020-12-19 14:33 "The video signal current output by the linear array CMOS is too small, and the input resistance of the AD module used is only 50ohm. I want to use an op amp to design a voltage follower, ...

Hello, my CMOS outputs a voltage signal, but the current is too small, only 1mA, and the driving ability is insufficient. Is the voltage signal after I/V conversion still the light intensity voltage signal output by my original CMOS?

S3S4S5S6

XuYong Xuyong published on 2020-12-19 14:58 Hello, my CMOS outputs a voltage signal, but the current is too small, only 1mA, and the driving capability is insufficient. The voltage signal after I/V conversion is still...

The output voltage signal is even easier to handle. Just amplify the voltage signal (just follow the circuit structure in the manual for the AD8066 in the previous figure, adjust the parameters and amplify it appropriately), then give it to the ADC, and then process it with the algorithm.

S3S4S5S6

XuYong Xuyong published on 2020-12-19 14:58 Hello, my CMOS outputs a voltage signal, but the current is too small, only 1mA, and the driving capability is insufficient. The voltage signal after I/V conversion is still...

I recommend you a chip I used before, the output current is 250mA, which is definitely enough

S3S4S5S6

XuYong Xuyong published on 2020-12-19 14:58 Hello, my CMOS outputs a voltage signal, but the current is too small, only 1mA, and the driving capability is insufficient. The voltage signal after I/V conversion is still...

TI chip BUF634

http://ti.com.cn/cn/lit/ds/symlink/buf634a.pdf?ts=1608311360696&ref_url=https%253A%252F%252Fwww.ti.com.cn%252Fproduct%252Fcn%252FBUF634A

XuYong虚庸

S3S4S5S6 Published on 2020-12-19 15:46 Output voltage signal is easier to handle, just amplify the voltage signal directly (the AD8066 in the previous figure can be directly adjusted according to the manual circuit structure, and...

OK, thank you very much for your detailed answer! I am considering using AD8066 to build a voltage follower and hope the output current can be larger.