Datasheet

www.ti.com
50W
50W
OPA365
40pF
100pF
1nF
50W
50W
OPA365
40pF
100pF
+IN
-IN
ADS8325
50W
50W
OPA365
40pF
100pF
50W
40pF
+IN
-IN
ADS8325
Single-Ended Differential
REFERENCE INPUT
3.0
2.5
2.0
1.5
1.0
0.5
0
-0.5
Delta(mV)
0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.750
V toV
DD REF
(V)
CHANGEINOFFSETANDGAINvs
SUPPLY/REFERENCEDIFFERENTIAL
Offset
Gain
ADS8325
SBAS226C – MARCH 2002 – REVISED AUGUST 2007
Figure 31. Single-Ended and Differential Methods of Interfacing the ADS8325
As the difference between the power-supply voltage
and reference voltage increases, the gain and offset
performance of the converter will decrease. Figure 32
The external reference sets the analog input range.
shows the typical change in gain and offset as a
The ADS8325 will operate with a reference in the
function of the difference between the power-supply
range of 2.5V to V
DD
. There are several important
voltage and reference voltage. For the combination of
implications to this.
V
DD
= 2.7V and V
REF
= 2.5V, or V
DD
= 5V and V
REF
=
As the reference voltage is reduced, the analog
5V, offset and gain error will be minimal. The most
voltage weight of each digital output code is reduced.
dramatic difference in offset can be seen when V
DD
=
This is often referred to as the Least Significant Bit
5V and V
REF
= 2.5V.
(LSB) size and is equal to the reference voltage
divided by 65,536. This means that any offset or gain
error inherent in the A/D converter will appear to
increase, in terms of LSB size, as the reference
voltage is reduced. For a reference voltage of 2.5V,
the value of LSB is 38.15 μ V, and for reference
voltage of 5V, the LSB is 76.3 μ V.
The noise inherent in the converter will also appear to
increase with lower LSB size. With a 5V reference,
the internal noise of the converter typically contributes
only 1.5LSBs peak-to-peak of potential error to the
output code. When the external reference is 2.5V, the
potential error contribution from the internal noise will
be 2 times larger (3LSBs). The errors due to the
internal noise are Gaussian in nature and can be
reduced by averaging consecutive conversion results.
For more information regarding noise, consult
Figure 9 , Peak-to-Peak Noise vs Reference Voltage.
Figure 32. Change in Offset and Gain vs the
Note that Figure 10 , Effective Number Of Bits vs
Difference Between Power-Supply and Reference
Input Frequency, is calculated based on the
Voltage
converter ’ s signal-to-(noise + distortion) ratio with a
1kHz, 0dB input signal. SINAD is related to ENOB as
follows:
SINAD = 6.02 × ENOB + 1.76
Copyright © 2002 – 2007, Texas Instruments Incorporated Submit Documentation Feedback 17
Product Folder Link(s): ADS8325