Datasheet

V 90=0.9(V )
REF REF_ADC
(5)
V 10=0.1(V )
REF REF_ADC
(6)
V 90=ADC atV 90
MEAS MEASUREMENT REF
(7)
V 10=ADC atV 10
MEAS MEASUREMENT REF
(8)
G =
MEAS
V 90 V 10-
MEAS MEAS
V 90 V 10-
REF REF
(9)
O =V 10 (V 10 G )- ´
MEAS MEAS REF MEAS
(10)
V =AnyV ADC
AD_MEAS IN MEASUREMENT
(11)
V =
ADC_CAL
V O-
AD_MEAS MEAS
G
MEAS
(12)
IdealTransferFunction
TransferFunction
withOffsetError+GainError
V =+5V
REF
OffsetError=+4LSB
GainError=+6LSB
0FFFh(4.99878V)
(4.5114751443V)
(0.5056191443V)
0000h(0V)
0V
V
IN
DigitalOutput(V )
AD_MEAS
0.5V
(0.1 V )´
REF_ADC
4.5V
(0.9 V )´
REF_ADC
4.99878V
(V 1LSB)
REF_ADC
-
PGA112 , , PGA113
PGA116 , PGA117
www.ti.com ............................................................................................................................................ SBOS424B – MARCH 2008 – REVISED SEPTEMBER 2008
The 12-bit ADC example in Figure 76 illustrates the
technique for calibrating an ADC using a
10%V
REF_ADC
and 90%V
REF_ADC
reading where
V
REF_ADC
is the ADC reference voltage. Note that the
10%V
REF
reading also contains a gain error because
it is not a V
IN
= 0 calibration point. First, use the
2. Compute the ADC measured gain. The slope of
90%V
REF
and 10%V
REF
points to compute the
the curve connecting the measured 10%V
REF
and
measured gain error. The measured gain error is then
measured 90%V
REF
point is computed and
used to remove the gain error from the 10%V
REF
compared to the slope between the ideal
reading, giving a measured 10%V
REF
number. The
10%V
REF
and ideal 90%V
REF
. This result is the
measured 10%V
REF
number is used to compute the
measured gain.
measured offset error.
3. Compute the ADC measured offset. The
measured offset is computed by taking the
difference between the measured 10%V
REF
and
the (ideal 10%V
REF
) × (measured gain).
4. Compute the calibrated ADC readings.
Any ADC reading can therefore be calibrated by
removing the gain error and offset error. The
measured offset is subtracted from the ADC reading
and then divided by the measured gain to give a
Figure 76. 12-Bit Example of ADC Calibration for
corrected reading. If this calibration is performed on a
Gain and Offset Error
timed basis, relative to the specific application, gain
and offset error over temperature are also removed
from the ADC reading by calibration.
The gain error and offset error in ADC readings can
be calibrated by using 10%V
REF_ADC
and
For example; given:
90%V
REF_ADC
calibration points. Because the
• 12-Bit ADC
calibration is ratiometric to V
REF_ADC
, the exact value
• ADC Gain Error = +6LSB
of V
REF_ADC
does not need to be known in the end
• ADC Offset Error = +4LSB
application.
• ADC Reference (V
REF_ADC
) = +5V
Follow these steps to compute a calibrated ADC
• Temperature = +25 ° C
reading:
1. Take the ADC reading at V
IN
= 90% × V
REF
and
Table 11 shows the resulting system accuracy.
V
IN
= 10% × V
REF
. The ADC readings for
10%V
REF
and 90%V
REF
are taken.
Table 11. Bits of System Accuracy
(1)
(to 0.5LSB)
ADC ACCURACY WITHOUT ADC ACCURACY WITH PGA112
V
IN
CALIBRATION CALIBRATION
10%V
REF_ADC
8.80 Bits 12.80 Bits
90%V
REF_ADC
7.77 Bits 11.06 Bits
(1) Difference in maximum input offset voltage for V
IN
= 10%V
REF_ADC
and V
IN
= 90%V
REF_ADC
is the reason for different accuracies.
Copyright © 2008, Texas Instruments Incorporated Submit Documentation Feedback 35
Product Folder Link(s): PGA112 PGA113 PGA116 PGA117