ADL5310

Rev. A | Page 16 of 20

CHARACTERIZATION METHODS

The solution in Figure 37 is no longer subject to potential

channel mismatch issues. Individual channel slope and intercept

characteristics can be calibrated independently. The accuracy

was verified using a pair of calibrated current sources. The

performance of the circuit depicted in Figure 37 is shown in

Figure 38 and Figure 39. Multiple transfer functions and error

plots are provided for various power levels. The accuracy is

better than 0.1 dB over a 5-decade range. The dynamic range is

limited available swing of the VLOG pin and can be recovered

through careful selection of input and output optical tap

coupling ratios.

During the characterization of the ADL5310, the device was

treated as a precision current-input logarithmic converter,

because it is impractical to generate accurate photocurrents by

illuminating a photodiode. The test currents were generated by

using either a well-calibrated current source, such as the

Keithley 236, or a high value resistor from a voltage source to

the input pin. Great care is needed when using very small input

currents. For example, the triax output connection from the

current generator was used with the guard tied to VSUM. The

input trace on the PC board was guarded by connecting

adjacent traces to VSUM.

1.0

1.2

1.4

1.6

1.8

0.8

0.6

0.4

0.2

0

–10

0

10

20

30

40

50

–20

60

α

φ

These measures are needed to minimize the risk of leakage

current paths. With 0.5 V as the nominal bias on the INP1

(INP2) pin, a leakage-path resistance of 1 GΩ to ground would

subtract 0.5 nA from the input, which amounts to a −1.6 dB

error for a 3 nA source current. Additionally, the very high

sensitivity at the input pins and the long cables commonly

needed during characterization allow 60 Hz and RF emissions

to introduce substantial measurement errors. Careful guarding

techniques are essential to reducing the pickup of these spurious

signals.

Additional information, including test setups, can be found in

the AD8305 and ADL5306 data sheets.

Figure 38. Absorbance and Absolute Power Transfer Functions for

Wilson Mirror ADL5310 Combination

0

0.1

0.2

0.3

0.4

0.5

–0.1

–0.2

–0.3

–0.4

–0.5

–30

–20

–10

0

10

20

30

–40

40

50

60

Figure 39. Log Conformance for Wilson Mirror ADL5310 Combination,