AD8305ACP-R2 datasheet(20/24 Pages) AD

20 / 24 page

AD8305

Data Sheet

Rev. C | Page 20 of 24

The setup shown in Figure 44 was used for frequency response

measurements of the logarithmic amplifier section. The AD8138

output is offset to 1.5 V dc and modulated to a depth of 5% at

frequency. R1 is chosen (over a wide range of values up to 1.0

GΩ) to provide IPD. The buffer was used to deload VLOG from

the measurement system.

0.1µF

SOURCE

CHANNEL 2

HP 89410A

1nF

1kΩ

1nF

200kΩ

ALKALINE

“D” CELL

ALKALINE

“D” CELL

+–

AD8305

COMM COMM COMM COMM

VSUM

VRDZ

VREF

IREF

INPT

VOUT

SCAL

BFIN

VLOG

VNEG VNEG VPOS

TRIGGER

CHANNEL 1

Figure 45. Configuration for Noise Spectral Density Measurement

The configuration in Figure 45 is used to measure the noise

performance. Batteries provide both the supply voltage and the

input current to minimize the introduction of spurious noise

and ground loop effects. The entire evaluation system, including

the current setting resistors, is mounted in a closed aluminum

enclosure to provide additional shielding to external noise

sources.

0.1µF

1nF 1kΩ

1kΩ

1nF

200kΩ

CH A

9213

LECROY 9210

CH1

TDS5104

+VS

AD8305

COMM COMM COMM COMM

VSUM

VRDZ

VREF

IREF

INPT

VOUT

SCAL

BFIN

VLOG

VNEG VNEG VPOS

Figure 46. Configuration for Logarithmic Amplifier Pulse Response

Measurement

Figure 46 shows the setup used to make the pulse response

measurements. As with the bandwidth measurement, the VLOG is

connected directly to BFIN and the buffer amplifier is configured

for unity gain. The output of the buffer is connected through a

short cable to the TDS5104 scope with input impedance set to

1 MΩ. The LeCroy’s output is offset to create the initial pedestal

current for a given value of R1, the pulse then creates one-decade

current step.

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