AD8310 データシートの表示（PDF） - Unspecified

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AD8310

Fast, Voltage-Out DC–440 MHz, 95 dB Logarithmic Amplifier

Unspecified 

APPLICATIONS

The AD8310 is highly versatile and easy to use. It needs only a

few external components, most of which can be immediately

accommodated using the simple connections shown in the

Using the AD8310 section.

A few examples of more specialized applications are provided in

the following sections. See the AD8307 data sheet for more

applications (note the slightly different pin configuration).

CABLE-DRIVING

For a supply voltage of 3 V or greater, the AD8310 can drive a

grounded 100 Ω load to 2.5 V. If reverse-termination is required

when driving a 50 Ω cable, it should be included in series with

the output, as shown in Figure 37. The slope at the load is then

12 mV/dB. In some cases, it might be permissible to operate the

cable without a termination at the far end, in which case the

slope is not lowered. Where a further increase in slope is

desirable, the scheme shown in Figure 34 can be used.

AD8310

50Ω

VOUT

50Ω

Figure 37. Output Response of Cable-Driver Application

DC-COUPLED INPUT

It might occasionally be necessary to provide response to dc

inputs. Because the AD8310 is internally dc-coupled, there is no

reason why this cannot be done. However, its differential inputs

must be positioned at least 2 V above the COM potential for

proper biasing of the first stage. Usually, the source is a single-

sided ground-referenced signal, so level-shifting and a single-

ended-to-differential conversion must be provided to correctly

drive the AD8310’s inputs.

Figure 38 shows how a level-shift to midsupply (2.5 V in this

example) and a single-ended-to-differential conversion can be

accomplished using the AD8138 differential amplifier. The four

499 Ω resistors set up a gain of unity. An output common-mode

(or bias) voltage of 2.5 is achieved by applying 2.5 V from a

supply-referenced resistive divider to the AD8138’s VOCM pin.

The differential outputs of the AD8138 directly drive the 1.1 kΩ

input impedance of the AD8310.

AD8310

0.01μF

SIGNAL

INPUT

5V

499Ω

499Ω

5V

0.1μF

AD8138

10kΩ

2.5V

10kΩ

499Ω

0.1μF 499Ω

NC

8

7

6

5

INHI ENBL BFIN VPOS

AD8310

INLO COMM OFLT VOUT

1

2

3

4

50Ω

1.87kΩ

3.01kΩ

NC = NO CONNECT

5V

VOUT

5V

Figure 38. DC-Coupled Log Amp

In this application the offset voltage of the AD8138 must be

trimmed. The internal offset compensation circuitry of the

AD8310 is disabled by applying a nominal voltage of ~1.9 V

to the OFLT pin, so the trim on the AD8138 is effectively

trimming the offsets of both devices. The trim is done by

grounding the circuit’s input and slightly varying the gain

resistors on the AD8138’s inverting input (a 50 Ω potentiometer

is used in this example) until the voltage on the AD8310’s

output reaches a minimum.

After trimming, the lower end of the dynamic range is limited

by the broadband noise at the output of the AD8138, which is

approximately 425 μV p-p. A differential low-pass filter can be

added between the AD8138 and the AD8310 when the very fast

pulse response of the circuit is not required.

2.7

2.5

2.3

2.1

1.9

1.7

1.5

1.3

1.1

0.9

0.7

0.1

1

10

100

INPUT LEVEL (mV)

1000

Figure 39. Transfer Function of DC-Coupled Log Amp Application

Rev. E | Page 19 of 24

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