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Help with ADC circuit - Anyone ever use LTC1298?

H

Harry Nguyen

Hello ,
Anyone ever use LTC1982 to build a ADC system. It's a 12bit serial
interface ADC chip from Linear Technology. They have an application
note that gives the schematic to couple the chip to PC for data
logging but when I do as they follow the result I get is very noisy.
Does anyone have experience dealing with this chip. Thanks a lot
Hope to hear from you
 
R

Rene Tschaggelar

Harry said:
Hello ,
Anyone ever use LTC1982 to build a ADC system. It's a 12bit serial
interface ADC chip from Linear Technology. They have an application
note that gives the schematic to couple the chip to PC for data
logging but when I do as they follow the result I get is very noisy.
Does anyone have experience dealing with this chip. Thanks a lot
Hope to hear from you

I didn't use this particular type yet. Apparently it is a n ADC with
capactive input. In order to get satisfying results you need the
caps over the supply. 100nF parallel 22uF. Plus you need the caps
at the input. Note that opamps do not like capacitive loads. So when you
drive the input with an opamp, make sure to have a series resistor.

Rene
 
J

Joerg

Hi Rene, Hi Harry,
... Note that opamps do not like capacitive loads. So when you drive
the input with an opamp, make sure to have a series resistor.

Or better yet, follow it with a sturdy buffer. Most ADCs need a nice low
drive impedance. Else digital noise might make it back into their
inputs. Sometimes that can even happen via paths on the chip that are
beyond a design engineer's control.

Other hint: Try a single solid ground plane. Breaking ground into AGND,
DGND and whatnot rarely works.

Regards, Joerg
 
R

Rene Tschaggelar

Joerg said:
Hi Rene, Hi Harry,



Or better yet, follow it with a sturdy buffer. Most ADCs need a nice low
drive impedance. Else digital noise might make it back into their
inputs. Sometimes that can even happen via paths on the chip that are
beyond a design engineer's control.

I once failed with that, when an ADC had a huge internal capacitance.
The Opamp thought to drive this input was oscillating.
It was solved with 1uF on the input and an 1k in series.
Luckily the bandwidth of the measurement was very low.
Imagine 5mV self generated spikes on the input of a 20 bit ADC.
All the newer ADCs are having capacitive inputs, as far as I could see.

Rene
 
J

Joerg

Hi Rene,
I once failed with that, when an ADC had a huge internal capacitance.
The Opamp thought to drive this input was oscillating.
It was solved with 1uF on the input and an 1k in series.
Luckily the bandwidth of the measurement was very low.
Imagine 5mV self generated spikes on the input of a 20 bit ADC.
All the newer ADCs are having capacitive inputs, as far as I could see.

Opamps, especially fast ones, generally do not like a capacitive load
and with a fast feedback loop they can become unstable. It is better to
use a buffer after the opamp. You could also stabilize it by
compensating for the capacitive load in the feedback path of the opamp.

5mV is a lot. Even with 10-12 bits you need to be lower than that if
your routines include Doppler signal processing.

Regards, Joerg
 
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