AD8555 Analog Devices, AD8555 Datasheet - Page 24

• Manufacturer Part Number: AD8555
• Description: Zero-Drift, Single-Supply, Sensor Signal Amplifier with Digitally Programmable Gain and Offset
• Manufacturer: Analog Devices

AD8555
Suggested Programming Procedure
1.
2.
3.
4.
Set VDD and VSS to the desired values in the application.
Use simulation mode to test and determine the desired
codes for the second stage gain, first stage gain, and output
offset. The nominal values for these parameters are shown
in Table 6, Table 7, Equation 1, and Equation 2; the codes
corresponding to these values can be used as a starting
point. However, since actual parameter values for given
codes vary from device to device, some fine tuning is nec-
essary for the best possible accuracy.
One way to choose these values is to set the output offset
to an approximate value, e.g., Code 128 for midsupply, to
allow the required gain to be determined. Then set the sec-
ond stage gain such that the minimum first stage gain
(Code 0) gives a lower gain than required, and the maxi-
mum first stage gain (Code 127) gives a higher gain than
required. After choosing the second stage gain, the first
stage gain can be chosen to fine tune the total gain. Finally,
the output offset can be adjusted to give the desired value.
After determining the desired codes for second stage gain,
first stage gain, and output offset, the device is ready for
permanent programming.
Set VSS to 0 V and VDD to 5.5 V. Use program mode to
permanently enter the desired codes for the second stage
gain, first stage gain, and output offset. Blow the master
fuse to allow the AD8555 to read data from the fuses and
to prevent further programming.
Set VDD and VSS to the desired values in the application.
Use read mode with low sense current followed by high
sense current to verify programmed codes.
Measure gain and offset to verify correct functionality.
Rev. 0 | Page 24 of 28
Suggested Algorithm to Determine
Optimal Gain and Offset Codes
1.
2a.
2b.
3a.
3b.
3c.
3d.
3e.
3f.
3g.
3h.
3i.
3j.
4a.
4b.
4c.
4d.
4e.
4f.
4g.
4h.
4i.
4j.
Determine the desired gain, G
Measure the resulting gain G
Measure the output offset O
ments).
Use Table 7 to determine the second stage gain G
that (4.00 × 1.04) < (G
that the first and last codes for the first stage gain are not
used, thereby allowing enough first stage gain codes
within each second stage gain range to adjust for the 3%
accuracy.
Use simulation mode to set the second stage gain to G
Set the output offset to allow the AD8555 gain to be
measured, e.g., use Code 128 to set it to midsupply.
Use Table 6 or Equation 3 to set the first stage gain code
C
3% of G
Calculate the first stage gain error (in relative terms)
E
Calculate the error (in the number of the first stage gain
codes) C
Set the first stage gain code to C
Measure the gain G
Calculate the error (in relative terms) E
Calculate the error (in the number of the first stage gain
codes) C
Set the first stage gain code to C
resulting gain should be within one code of G
Determine the desired output offset O
measurements.
Use Equation 1 to set the output offset code C
that the output offset is nominally O
3% of O
Calculate the error (in relative terms) E
Calculate the error (in the number of the output offset
codes) C
Set the output offset code to C
Measure the output offset O
than to O
Calculate the error (in relative terms) E
Calculate the error (in the number of the output offset
codes) C
Set the output offset code to C
resulting offset should be within one code of O
G1
G1
= G
such that the first stage gain is nominally G
B
A
A
/G
EG1
EG2
EO1
EO2
.
.
B
.
A
= E
= E
= E
= E
− 1.
G1
G2
O1
O2
/0.00370.
/0.00370.
/0.00392.
/0.00392.
C
. G
C
A
/G
should be closer to G
2
) < (6.4/1.04). This ensures
B
C
B
. O
. O
. G
A
O1
O1
(e.g., using measure-
B
C
G1
G1
B
− C
− C
should be within
should be closer to O
should be within
− C
− C
EO1
EO1
A
.
EG1
EG1
A
.
G2
, e.g., using the
O1
O2
− C
.
− C
= G
= O
= O
EO2
A
EG2
than to G
C
B
C
A
O1
. The
/G
/O
/O
.
A
A
. The
.
/G
such
2
A
A
A
such
− 1.
− 1.
2
− 1.
.
2
B
.
A
.