ADL5310-EVAL AD [Analog Devices], ADL5310-EVAL Datasheet - Page 15

• Manufacturer Part Number: ADL5310-EVAL
• Description: 120 dB Range (3 nA to 3 mA) Dual Logarithmic Converter
• Manufacturer: AD [Analog Devices]

A 10 nF capacitor on each VSUM pin (20 nF parallel equivalent)
combined with the 16 kΩ source resistance yields a 500 Hz pole,
which is sufficiently below the bandwidth for the minimum
input current of 3 nA.
Residual crosstalk disturbance is particularly problematic at the
lowest currents for two reasons. First, the loop is unable to reject
summing node disturbances beyond the limited bandwidth.
Second, the settling response at the lowest currents to any
residual disturbance is significantly slower than that for input
currents even one or two decades higher (see Figure 18).
Figure 36 shows the measured response of an inactive channel
(dc input) to a 1-decade current step on the input of the active
channel for several inactive channel dc current values. Addi-
tional system considerations may be necessary to ensure
adequate settling time following a known transient when one or
both channels are operating at very low input currents.
RELATIVE AND ABSOLUTE POWER
MEASUREMENTS
When properly calibrated, the ADL5310 provides two inde-
pendent channels capable of accurate absolute optical power
measurements. Often, it is desirable to measure the relative
gain or absorbance across an optical network element, such as
an optical amplifier or variable attenuator. If each channel has
identical logarithmic slopes and intercepts, this can easily be
done by differencing the output signals of each channel. In
reality, channel mismatch can result in significant errors over a
wide range of input levels if left uncompensated. Postprocessing
of the signal can be used to account for individual channel
characteristics. This requires a simple calculation of the
expected input level for a measured log voltage, followed by
differencing of the two signal levels in the digital domain for a
Figure 36. Crosstalk Pulse Response for Various Input Current Values
12
–3
–6
9
6
3
0
0
INACTIVE CHANNEL RESPONSE
ACTIVE CHANNEL OUTPUT PULSE, 1-DECADE STEP
3µA TO 30µA
I
INP
I
INP
– 30nA
0.5
– 3nA
I
INP
– 10nA
1.0
TIME (ms)
1.5
I
INP
– 100nA
2.0
2.5
1.2
1.0
0.8
0.6
0.4
0.2
0
Rev. A | Page 15 of 20
relative gain or absorbance measurement. A more straight-
forward analog implementation includes the use of a current
mirror, as shown in Figure 37. The current mirror is used to
feed an opposite polarity replica of the cathode photocurrent of
PD2 into Channel 2 of the ADL5310. This allows one channel to
be used as an absolute power meter for the optical signal
incident on PD2, while the opposite channel is used to directly
compute the log ratio of the two input signals.
The presented current mirror is a modified Wilson mirror.
Other current mirror implementations would also work, though
the modified Wilson mirror provides fairly constant perfor-
mance over temperature. It is essential to use matched pair
transistors when designing the current mirror to minimize the
effects of temperature gradients and beta mismatch.
InGaAs PIN
5V
InGaAs PIN
0.1µF
PD2
0.1µF
PD1
Figure 37. Absolute and Relative Power Measurement Application
5V
I
IN2
1kΩ
2MΩ
4.7nF
=I
1kΩ
1kΩ
1kΩ
4.7nF
4.7nF
4.7nF
I
I
PD2
PD2
IN1
1nF
1nF
VSUM
IRF2
VREF
VSUM
INP2
VRDZ
IRF1
INP1
Using Modified Wilson Current Mirror
log
log
log
log
ADL5310
**α
*Φ
21
2
(V) ≅ 0.2log
(V) ≅ 0.2log
COMPENSATION
COMPENSATION
TEMPERATURE
TEMPERATURE
VNEG
GENERATOR
BIAS
VPOS
10
10
COMM
(
(
100pA
5V
I
I
I
PD2
IN2
IN1
COMM
COMM
)
I
I
LOG2
LOG1
)
ADL5310
LOG1
LOG2
OUT1
SCL1
OUT2
SCL2
BIN1
BIN2
1nF
1nF
α
Φ
21
2
*
**