AD8555ACPZ-R21 datasheet(24/28 Pages) AD

24 / 28 page

AD8555

Rev. A | Page 24 of 28

Suggested Programming Procedure

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 ne-

cessary 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

second 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.

Suggested Algorithm to Determine

Optimal Gain and Offset Codes

Determine the desired gain, GA (e.g., using measure-

ments).

2a.

Use Table 7 to determine the second stage gain G2 such

that (4.00 × 1.04) < (GA/G2) < (6.4/1.04). This ensures

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.

2b.

Use simulation mode to set the second stage gain to G2.

3a.

Set the output offset to allow the AD8555 gain to be

measured, e.g., use Code 128 to set it to midsupply.

3b.

Use Table 6 or Equation 3 to set the first stage gain code

CG1 such that the first stage gain is nominally GA/G2.

3c.

Measure the resulting gain GB. GB should be within

3% of GA.

3d.

Calculate the first stage gain error (in relative terms)

EG1 = GB/GA − 1.

3e.

Calculate the error (in the number of the first stage gain

codes) CEG1 = EG1/0.00370.

3f.

Set the first stage gain code to CG1 − CEG1.

3g.

Measure the gain GC. GC should be closer to GA than to GB.

3h.

Calculate the error (in relative terms) EG2 = GC/GA − 1.

3i.

Calculate the error (in the number of the first stage gain

codes) CEG2 = EG2/0.00370.

3j.

Set the first stage gain code to CG1 − CEG1 − CEG2. The

resulting gain should be within one code of GA.

4a.

Determine the desired output offset OA, e.g., using the

measurements.

4b.

Use Equation 1 to set the output offset code CO1 such

that the output offset is nominally OA.

4c.

Measure the output offset OB. OB should be within

3% of OA.

4d.

Calculate the error (in relative terms) EO1 = OB/OA − 1.

4e.

Calculate the error (in the number of the output offset

codes) CEO1 = EO1/0.00392.

4f.

Set the output offset code to CO1 − CEO1.

4g.

Measure the output offset OC. OC should be closer to OA

than to OB.

4h.

Calculate the error (in relative terms) EO2 = OC/OA − 1.

4i.

Calculate the error (in the number of the output offset

codes) CEO2 = EO2/0.00392.

4j.

Set the output offset code to CO1 − CEO1 − CEO2. The

resulting offset should be within one code of OA.

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