Lpc2000

Has anyone used lpc's to operate AD7677s (using lpc's) before?

- 16-bit True differential input (-2.5V to +2.5V input range)
- 1 MSPS conversion rate

Basically the ADC's set up such that:
3V                                ---> Vref pin
Analog input (always +ve Voltage) ---> Vin+ pin
-90mV reference                   ---> Vin- pin

I have a little problem with the readings that I'm currently getting:

Input = 0V (GND)
The lpc reads 0x8000 from the converter. I assume this is the mid-
point.

Input = 330mV
Reading: 0x8000....strange
Input = 660mV
Reading = 0x8050...stranger
Input = 1.32V
Reading = 0xB8F0. OK (the calculation's legit)

The A/D seems to be reading correct but only for a specific range. 
1.32V and above will read correctly. But For some reason the lower 
voltages aren't coming out right. Any ideas? Could the hardware 
setup for Vin+ and Vin- be causing this problem?

thanks,
Leighton

It appears there is a common mode voltage problem. Check if there is a 
current return path between AIN-/AIN+ and AGND. What you observed can happen 
if the signal source is floating with respect to AGND. As you have noted the 
AD accepts -2.5V to 2.5V. But since the part is not bipolar, the applied 
input signal must not be above VCC or below ground with respect to AGND. 
i.e, -2.5V actually means 0V connected to AIN+ and 2.5V connected to AIN-. 
It is not -2.5V with respect to AGND.

Just make sure voltage on any of the differential pins is in between the 
part's common mode range.

Kerem


----- Original Message ----- 

From: "Leighton Rowe" <leightonsrowe@...>
To: <lpc2000@yahoogroups.com>
Sent: Thursday, September 16, 2004 12:08 AM
Subject: [lpc2000] Using External AD Converters


> Has anyone used lpc's to operate AD7677s (using lpc's) before?
>
> - 16-bit True differential input (-2.5V to +2.5V input range)
> - 1 MSPS conversion rate
>
> Basically the ADC's set up such that:
> 3V                                ---> Vref pin
> Analog input (always +ve Voltage) ---> Vin+ pin
> -90mV reference                   ---> Vin- pin
>
> I have a little problem with the readings that I'm currently getting:
>
> Input = 0V (GND)
> The lpc reads 0x8000 from the converter. I assume this is the mid-
> point.
>
> Input = 330mV
> Reading: 0x8000....strange
> Input = 660mV
> Reading = 0x8050...stranger
> Input = 1.32V
> Reading = 0xB8F0. OK (the calculation's legit)
>
> The A/D seems to be reading correct but only for a specific range.
> 1.32V and above will read correctly. But For some reason the lower
> voltages aren't coming out right. Any ideas? Could the hardware
> setup for Vin+ and Vin- be causing this problem?
>
> thanks,
> Leighton
>
>
>
>
>
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
>

The analog inputs and references look stable to me. No floating 
pins. I've made the Vin- input closer to ground:

Analog input (always +ve)---> Vin+ 
0V                       ---> Vin-
3V                       ---> Vref
5V                       ---> AVDD/DVDD

...but the same readings are still coming out. Maybe it's because 
I'm using a "true differential" ADC. I heard the setup's different 
compared to pseudo differential which I'm more accustomed to using.

--- In lpc2000@yahoogroups.com, "Kerem Or" <k.or@s...> wrote:
> It appears there is a common mode voltage problem. Check if there 
is a 
> current return path between AIN-/AIN+ and AGND. What you observed 
can happen 
> if the signal source is floating with respect to AGND. As you have 
noted the 
> AD accepts -2.5V to 2.5V. But since the part is not bipolar, the 
applied 
> input signal must not be above VCC or below ground with respect to 
AGND. 
> i.e, -2.5V actually means 0V connected to AIN+ and 2.5V connected 
to AIN-. 
> It is not -2.5V with respect to AGND.
> 
> Just make sure voltage on any of the differential pins is in 
between the 

> part's common mode range.
> 
> Kerem

It sounds like your inputs aren't truly differential.  You've 
referenced the V- input as a reference 90mV.... which makes it a 
unipolar signal, essentially.  A differential input won't have this 
type of reference as Karem mentioned below.   You could force the 
inputs into the 0-5V range by using a 2.5V ADC reference.  You can 
still you a differential input but will then want to make the input 
amplifier's common mode reference at 2.5V... or half of the reference 
voltage so that it's centered in the range.  
Edwin

--- In lpc2000@yahoogroups.com, "Leighton Rowe" <leightonsrowe@y...> 
wrote:
> The analog inputs and references look stable to me. No floating 
> pins. I've made the Vin- input closer to ground:
> 
> Analog input (always +ve)---> Vin+ 
> 0V                       ---> Vin-
> 3V                       ---> Vref
> 5V                       ---> AVDD/DVDD
> 
> ...but the same readings are still coming out. Maybe it's because 
> I'm using a "true differential" ADC. I heard the setup's different 
> compared to pseudo differential which I'm more accustomed to using.
> 
> --- In lpc2000@yahoogroups.com, "Kerem Or" <k.or@s...> wrote:
> > It appears there is a common mode voltage problem. Check if there 
> is a 
> > current return path between AIN-/AIN+ and AGND. What you observed 
> can happen 
> > if the signal source is floating with respect to AGND. As you 
have 
> noted the 
> > AD accepts -2.5V to 2.5V. But since the part is not bipolar, the 
> applied 
> > input signal must not be above VCC or below ground with respect 
to 

> AGND. 
> > i.e, -2.5V actually means 0V connected to AIN+ and 2.5V connected 
> to AIN-. 
> > It is not -2.5V with respect to AGND.
> > 
> > Just make sure voltage on any of the differential pins is in 
> between the 
> > part's common mode range.
> > 
> > Kerem

Thanks

--- In lpc2000@yahoogroups.com, "ebland_us" <ebland_us@y...> wrote:
> It sounds like your inputs aren't truly differential.  You've 
> referenced the V- input as a reference 90mV.... which makes it a 
> unipolar signal, essentially.  A differential input won't have 
this 
> type of reference as Karem mentioned below.   You could force the 
> inputs into the 0-5V range by using a 2.5V ADC reference.  You can 
> still you a differential input but will then want to make the 
input 
> amplifier's common mode reference at 2.5V... or half of the 
reference 
> voltage so that it's centered in the range.  
> Edwin
> 
> --- In lpc2000@yahoogroups.com, "Leighton Rowe" 
<leightonsrowe@y...> 
> wrote:
> > The analog inputs and references look stable to me. No floating 
> > pins. I've made the Vin- input closer to ground:
> > 
> > Analog input (always +ve)---> Vin+ 
> > 0V                       ---> Vin-
> > 3V                       ---> Vref
> > 5V                       ---> AVDD/DVDD
> > 
> > ...but the same readings are still coming out. Maybe it's 
because 
> > I'm using a "true differential" ADC. I heard the setup's 
different 
> > compared to pseudo differential which I'm more accustomed to 
using.
> > 
> > --- In lpc2000@yahoogroups.com, "Kerem Or" <k.or@s...> wrote:
> > > It appears there is a common mode voltage problem. Check if 
there 
> > is a 
> > > current return path between AIN-/AIN+ and AGND. What you 
observed 
> > can happen 
> > > if the signal source is floating with respect to AGND. As you 
> have 
> > noted the 
> > > AD accepts -2.5V to 2.5V. But since the part is not bipolar, 
the 
> > applied 
> > > input signal must not be above VCC or below ground with 
respect 
> to 
> > AGND. 
> > > i.e, -2.5V actually means 0V connected to AIN+ and 2.5V 
connected 

> > to AIN-. 
> > > It is not -2.5V with respect to AGND.
> > > 
> > > Just make sure voltage on any of the differential pins is in 
> > between the 
> > > part's common mode range.
> > > 
> > > Kerem