Questions about AD8042AN - gain problems

[oliverlin09]

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.

 

[karesz]

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.

 

[FvM]

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.

 

[oliverlin09]

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

 

[FvM]

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.

 

[karesz]

Re: Questions about AD8042AN

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.

 

[oliverlin09]

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

 

[FvM]

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.

 

[oliverlin09]

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??

 

[LvW]

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??

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.

 

[oliverlin09]

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.

 

[LvW]

Re: Questions about AD8042AN

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.

 

[oliverlin09]

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???

 

[FvM]

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.

 

[LvW]

Re: Questions about AD8042AN

................
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!