Encode needed Voltage  for an AD9057

It's on page 2 of the datasheet.

-40 part is 40MHz max
-60 part is 60MHz max
-80 part is 80MHz max

Minimum in each case is 5MHz.

Clock levels are TTL.

There you have it, JC.  This is some kind of fancy-schmancy RF ADC that is AC coupled.  Our hero is having a difficulty slewing his pot at a 5MHz rate!

@ Don, So funny - but it would help the OP more if you read the data sheet.

He says "AD9057  using DC coupling exactly as described in the datasheet"  -that'll be figure 3 on page 7, "DC coupled AD9057 (inverted Vin)".

@ Jon - I cant see a bottom limit for input frequency on page 2  of the spec - but I can see the DC accuracy spec and the 5MHz minimum conversion rate.  So the part can measure DC, but needs to convert at at least 5MHz.

So, @ Imed - how are you driving the conversion input, (Encode on pin 10) and how are you reading the data out ?

(The best way to get help will be for you to post a complete schematic of your test set up.)

MK

Sorry, I didn't make it clear - I was answering the question about the voltage or frequency of the encode input (it's really a poorly named clock input).

As you say, the analogue input goes down to DC. It's a flash converter intended for digitising video (amongst other things), so if it didn't go down to DC it would need an internal clamp for DC restoration.

Thank you so much for your answers guys 

@ Michael Well I am using AD9057 (60 MSPS), you find attached the schematic of my board, I meant by " I am using DC coupling exactly described in the datasheet " that I used the circuit design exactly as the Evaluation board schematic p10 in the datasheet, I just replaced the BNCs by pinheaders, and I havent used the DAC Converter.

Where is it mentioned that the minimum DC coupling conversion rate is 5 Mhz please ? well as I understand it, since I am applying a constant analog input voltage, then the encode frequency doesn t matter that much, thats why asked about the minimum voltage that should be on ENC (pin 10), am I wrong?

I am driving the Encode input (Pinheader Encode  input described on the schematic; not ENC pin 10 ) with a blinking LED ( frequency is 1 Hz, voltage at TTL "HD74LS00"  inputs 1 and 2  is  between 0.2V to 4.7V ) but the output of the TTL (pin3 ) which is connected to my ENC ( pin10 of ADC) is constant ( 0.15 V ).

I am using LEDs to read my output data as well.

Would it make sense to apply a square signal from a signal generator with 5 Mhz to the ENCODE pinheader input ? if yes, how much should the amplitude be please ?

if you need more info please let me know.

Hello Imed, not much time to answer you now so I haven't checked your circuit in detail.

On page 2 of the AD datasheet under section "SWITCHING PERFORMANCE" it states Minimum Conversion Rate = 5MHz.

So you need to increase your led blink rate to 5MHz.

Remove R7 and R8 which won't be doing you much good - at some stage adding a resistor in series with IC1A inputs might help with signal shapes but at 5MHz it should be OK.

Apply a 0-5V pulse train to the encode input at a repetition rate of 5MHz.

So long as you have your polarities correct your latch should catch the ADC output and show it on the LEDs,  I haven't checked the timing.

Why did you use such a high speed converter  - do you plan to run it faster once it's working ?

MK

Hello Michael

ok I see, do you mean I should apply a 0-5V pulse train to the encode input ( Pinheader input or ENC of the ADC "pin 10" )  ?

so  with DC coupling  I should apply   0V to 0.5V max ( from the potentiometer ) to AIN , and I should get as output ( 00000000) for 0V and (11111111) for 0.5V, and something in between for example (10000000) for 0.25 V, to make sure that the ADC is working alright,am I right?

yes I want to run it faster once it s working , so will it affect the performance of the ADC if I leave R7 and R8 for other purposes i.e higher signal frequencies ?

Hint : I am using "MC74AC573" latch.

regards

Imed

The termination with R7/R8 is wrong for any frequency (a single 50R resistor to ground would do but you will have trouble finding  a signal generator that will do 0 - 10V pulses from 50R at 40MHz)

For high frequency working 5V logic is a very poor choice  - just get it working at 5MHz first and worry about speeding it up later.

The amplifier is an inverter so you need to apply -0.5 to +0.5V to the input to swing adc output from 0x00 to 0xff.

AD have used 74AC logic to get the required speed - your schematic shows 74HC logic which won't work.

MK

Here is a design provided by the good folks at BitScope A/D Buffer

ok I see, yeah I am using MC74AC573 logic so no worries about that, so as an abstract of what you mentioned, I should remove R7 and R8 completely from the circuit and apply 0-5V with a frequency of minimum 5 MHz to the Encode input pinheader, and then apply an input voltage -0.5V to +0.5 V to swing ADC output from 0x00 to 0xFF. is that right ?

But I should still get a clear pattern of the ADC output  if I apply a constant voltage to AIN and change it manually from the potentiometer ( 0 - +0.5V)  right?

regards

Imed