Lpc2000

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
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