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Questions about AD8042AN - gain problems

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oliverlin09

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Questions about AD8042AN

Hi guys,

I bought some AD8042AN chips, and tested them with the attached circuit.
I found a strange thing.
From the data sheet, that chip should have 160MHz bandwidth.
However, I input an 1MHz-100mV signal, I got the output was just 4V.
So my circuit was gain 99, but actually I just got gain 40.
I changed C1=33p and R2=10K; so the circuit had gain 10.
I input a 10MHz-100mV signal, but I just got 300mV from the output.
So I used an 1MHz signal, the bandwidth I got from the chip was 40MHz.
And when I used a 10MHz signal, the bandwidth I got from the chip was just 30MHz.

I am wondering if anybody has this kind of problems.
 

Questions about AD8042AN

Hi oliverlin09,
Your only problem is, that you have tooo high resistor values selected, THEY ARE NOT high bandwidth conform:-(...

Check pls the data sheet schematics or Appl. Notes from ADI.
Rf(your R2) is self in the Video Line Driver only 1KOhm, & these is on pp2 only 200 Ohm for the specified min. 125 MHz bandwidth!!
Also change pls. Rf (=R2) to 1K Ohm if you need the bandwidth and calculate an R1 for your application...
Other question is, that your Input Imp. will be (very) low_ the appl. examples are not randomly non inverting circuits_ here is the Inp. Imp. theoretically infinite is.

On 10KOhm +(minimum)33pF >>~50pF (+ i.e. 10pF &1 10pF of your probe_dont forget parasitics/stray Capacitances of board too)= is a lowpass of max. ~5MHz(3dB):-(...

WHAT IS your LOAD & SIGNAL FORM pls, did you measured with a scope probe?
Some thing is suspect/wrong form your measurements...

I can not understand your 40 & 30 MHz bandwidths to 1 & 10 MHz input signals! What do you means?
K.
 
Re: Questions about AD8042AN

I see two points:
- C1 = 33p creates a high pass corner above 1 MHz.

- You are confusing observed bandwidth at specific frequency and gain-bandwidth-product. You can see from the datasheet,
that the gain-bandwidth-product is about 100 MHz rather than 160 MHz. The slogan-like 160 MHz unity gain bandwidth
is only achieved with the help of severe peaking. Read the datasheet!

- It's not reasonable to use OP's with <1 loop gain.

Having a gain-bandwidth product of 100 MHz, the closed loop gain at 1 MHz can be expected lower than 100. OPs have been
designed to achieve stable gain utilizing feedback. But some loop gain is required for the concept.
 
Re: Questions about AD8042AN

Hiya,

As FvM said, I think I confus the bandwidth and the gain-bandwidth-product. So that is why I said I got 40MHz bandwidth (Frequency * Gain) when I used 1MHz signal; and 30MHz bandwidth when I used 10MHz signal.

karesz, my filter is a high pass filter and the signal is a sine wave. Basically I just want to use 10MHz sine wave signal, other frequencies are just for testing.

I just changed my circuit - C1=1n, R1=60.4 and R2=1K. I input 10MHz-100mV, and I could get 680mV now. (Currently, I just have those components, I may need to buy some resistors and capacitors in different values tomorrow)

Do you have any sugguestions to optimise my circuit???

Oliver
 

Re: Questions about AD8042AN

C1=1n, R1=60.4 and R2=1K. I input 10MHz-100mV, and I could get 680mV now.
In the last case, about 1V could be expected according to gain-bandwidth product. So there is a discrepancy. But how do you
measure the voltages? Do you observe in- and output voltages simultaneaously? How did you build the circuit? 10 MHz won't
work very good on a breadboard.
Do you have any sugguestions to optimise my circuit?
Optimize to where? It's not actually clear what you want to achieve.
 

Re: Questions about AD8042AN

oliverlin09 said:
Hiya,
...karesz, my filter is a high pass filter and the signal is a sine wave. Basically I just want to use 10MHz sine wave signal, other frequencies are just for testing...
Oliver
Yes, sorry your input HP-filtering is clear me, but I spoke (mistakly)over 33pF(+) parallel to Rf/R2...
K.
 

Re: Questions about AD8042AN

Hi FvM,

Yes, I built my circuit on a breadboard, because it's easier to change components.
I used a oscilloscope to see my frequency, waveform and Pk-Pk voltage (amplitude). And my signal source was from a function generator.
As you said, perhaps that was why I couldn't get 1V output signal.

The meaning of "optimise" was trying to get 10MHz-100mV input signal to have around 1V output.

Oliver
 

Re: Questions about AD8042AN

Basically you can trust the ADI datasheets. As I already mentioned, I don't see much sense in operating an OP without
effective feedback at it's open loop gain. If you need x10 gain @ 10 MHz, either use two cascaded OPs or a higher GBW type.

A suitable assembly technique is required however. RF prototype PCBs with a continuous ground plane are fine
for this purpose.
 

Re: Questions about AD8042AN

AD8042 has 2 op amps.
That circuit is for the first amp and its output will go to the 2nd amp; and I will build a multi-feedback band-pass circuit for the second one.
If there are anything I need to be careful for the multi-feedback band-pass circuit??
 

Re: Questions about AD8042AN

oliverlin09 said:
AD8042 has 2 op amps.
That circuit is for the first amp and its output will go to the 2nd amp; and I will build a multi-feedback band-pass circuit for the second one.
If there are anything I need to be careful for the multi-feedback band-pass circuit??

May I jump into the discussion?
There are two points to comment:
* What means "first opamp"? With or without feedback? If yes, which gain?
* A multi-feedback filter needs an opamp with a gain-bandwidth product which is at least 100 times greater than the wanted pole frequency!

It would be much easier to help if you would show a circuit diagram.
 

Re: Questions about AD8042AN

The 1st op amp is shown as AD8042A on the attached file.
It doesn't have feedback.

I only used equations to get values of resistors and capacitors for the 2nd part (AD8042B that bit).
I haven't tested it yet.
So I am not sure if it works.
 

Re: Questions about AD8042AN

oliverlin09 said:
The 1st op amp is shown as AD8042A on the attached file.
It doesn't have feedback.
..........

It doesn`t have feedback?
I see a negative feedback path resulting in a gain of app. -16.

In principle, the circuitry looks quite OK (assuming the same power supply also for the 2nd stage). However, the opamp choice (sufficient gain-bandwidth) is another point.
 

Re: Questions about AD8042AN

Yes, the 2nd op amp uses the same power supply as the 1st one, because AD8042 just has one +V and one -V to supply 2 amps.

I test the 1st stage last nite.
I input 100mV @ 10MHz, but I just got 680mV.
For that design, I should have got something around 1V.
I am wondering if I need to change R5 and R6 to be 1K or 2K.
Is it helpful for my circuit???
 

Questions about AD8042AN

You'll face considerable degradation of the filter characteristic due to limited OP GBW, e.g. shift of the bandpass center
frequency from 10 MHz to about 7 MHz and reduced gain.
 

Re: Questions about AD8042AN

oliverlin09 said:
................
I input 100mV @ 10MHz, but I just got 680mV.
For that design, I should have got something around 1V.

You cannot get everything you want (I should have got...).
The result you´ve got is OK for the chosen opamp which has at 10MHz an open-loop gain of only 20 dB!. All the simplified gain formulas do not apply anymore!
 

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