Data Sheet

ADL5303

Rev. 0 | Page 17 of 24

APPLICATIONS INFORMATION

Smaller input voltages can be measured accurately when

aided by a small offset nulling voltage applied to VSUM.

The minimum voltage that can be accurately measured is

limited only by the drift in the input offset of the ADL5303.

The specifications show the maximum spread over the full

temperature and supply range. Over a limited temperature

range and with a regulated supply, the offset drift is lower;

in this situation, processing of inputs down to 5 mV is

practicable.

RESCALING

The use of a much larger value for the intercept may be useful

in certain situations. In this example, it has been moved up four

decades, from the default value of 100 pA to the center of the

full eight-decade range at 1 mA. Using a voltage input as previ-

ously described, this corresponds to an altered voltage mode

of the larger output swing, the gain of the buffer has been

increased to 4.53, resulting in a scaling of 900 mV/decade

zand a full-scale output of ±3.6 V.

INVERTING THE SLOPE

The buffer is essentially an uncommitted op amp that can be

used to support the operation of the ADL5303 in a variety of

ways. It can be completely disconnected from the signal chain

when not needed. Figure 30 shows its use as an inverting ampli-

fier; this changes the polarity of the slope. The output can be

to the BFIN pin. The full design for a practical application is

left undefined in this brief illustration, but a few cases are

discussed, as follows.

For example, if slope of −30 mV/dB is needed; a gain of 3 is

required. Because VLOG exhibits a source resistance of 5 kΩ,

pin, as indicated in Figure 30. The resulting output voltage can

be expressed as

OS

Z

PD

Y

A

OUT

V

I

I

V

R

V

+













×

















−

=

10

log

kΩ

5

(11)

directly to BFIN, in which case the starting point for the output

response is at 4 V. However, because the slope in this case is

only −0.2 V/decade, the full current range takes the output

down by only 1.6 V. Clearly, a higher slope (or gain) is desirable;

starts at 4.8 V and falls through this same voltage toward ground

NC

NC

NC = NO CONNECT

100nF

R1

750

Ω

C1

1nF

C3

PDB

BIAS

VREF

VPDB

VSUM

INPT

VSUM

ACOM

VPS2

PWDN

VPS1

VREF

VLOG

BFIN

BFNG

VOUT

0.5V

ADL5303

~10k

Ω

5k

Ω

5

2

3

4

15

14

GND

GND

7

11

10

16

12

6

8

9

13

TEMPERATURE

COMPENSATION

Figure 30. Using the Buffer to Invert the Polarity of the Slope