AD8310

Rev. F | Page 19 of 24

APPLICATIONS INFORMATION

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

VOUT

50

Ω

50

Ω

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 VOCM pin of the AD8138. The

differential outputs of the AD8138 directly drive the 1.1 kΩ

input impedance of the AD8310.

5V

0.01 F

μ

SIGNAL

INPUT

AD8138

0.1

μF

5V

499

Ω

499

Ω

499

Ω

499

Ω

10k

Ω

0.1

μF

5V

10k

Ω

NC

INHI ENBL BFIN VPOS

INLO COMM OFLT VOUT

AD8310

12

34

87

65

VOUT

5V

3.01k

Ω

1.87k

Ω

50

Ω

2.5V

NC = NO CONNECT

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

inverting input of the AD8138 (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.

INPUT LEVEL (mV)

0.1

1

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

10

100

1000

2.3

2.5

2.7

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