AD8557ACPZ-R2 datasheet(20/24 Pages) AD

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AD8557

Rev. C | Page 20 of 24

Determining Optimal Gain and Offset Codes

First, determine the desired gain:

1. Determine the desired gain, GA (using the measurements

obtained from the simulation).

2. Use Table 7 to determine G2, the second stage gain, such

that (2.8 × 1.05) < (GA/G2) < (5.2/1.05). This ensures 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.

Next, set the second stage gain:

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

2. Set the output offset to allow the AD8557 gain to be

measured, for example, use Code 128 to set it to midsupply.

3. Use Table 6 or Equation 1 to set the first stage gain code

CG1, so the first stage gain is nominally GA/G2.

4. Measure the resulting gain (GB). GB should be within

3% of GA.

5. Calculate the first stage gain error (in relative terms)

EG1 = GB/GA − 1.

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

codes) CEG1 = EG1/0.00489.

7. Set the first stage gain code to CG1 − CEG1.

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

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

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

codes) CEG2 = EG2/0.00489.

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

resulting gain should be within one code of GA.

Finally, determine the desired output offset:

1. Determine the desired output offset OA (using the

measurements obtained from the simulation).

2. Use Equation 2 to set the output offset code CO1 such that

the output offset is nominally OA.

3. Measure the output offset (OB). OB should be within

3% of OA.

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

5. Calculate the error (in the number of the output offset

codes) CEO1 = EO1/0.00392.

6. Set the output offset code to CO1 − CEO1.

7. Measure the output offset (OC). OC should be closer to OA

than to OB.

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

9. Calculate the error (in the number of the output offset

codes) CEO2 = EO2/0.00392.

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

resulting offset should be within one code of OA.

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