Data Sheet

AD8338

Rev. 0 | Page 13 of 16

For example, if the 500 Ω input resistors and 9.5 kΩ feedback

to 0.1 V, the output value is as follows:

(5a)

(5b)

(5c)

The calculation in Equation 5 results in a total gain of approx-

imately −0.4 dB under the specified conditions. Compressing

Equation 2 through Equation 4 produces the following simplified

gain equation:

(6)

input to a single output.

INPR

INPD

INMD

INMR

MODE

COMM

GAIN

DETO

VAGC

VGA CORE

–26dB TO +54dB

OFFSET NULL

OFSN

VREF

VBAT

AUTOMATIC

GAIN

CONTROL

GAIN INTERFACE

AD8338

9.5kΩ

9.5kΩ

VREF

FBKP

OUTP

OUTM

FBKM

500Ω

500Ω

Figure 40. Functional Block Diagram

For example, if a design requires a minimum gain of 20 dB using

a few additional components, Equation 6 shows that applying a

47 Ω resistor to both the INPD and INMD pins (overriding the

INPR

INPD

INMD

INMR

500Ω

500Ω

47Ω

47Ω

20dB TO 100dB

OUTP

OUTM

Figure 41. Using External Resistors at the INPD and INMD Pins

Similarly, if the user requires a minimum gain of −10 dB,

applying a 1.5 kΩ resistor to both the INPD and INMD pins

sets a gain range of −10 dB to +70 dB.

Effects of Using External Resistors

When the gain is modified through the use of external resistors,

several trade-offs must be considered. For example, with the appli-

cation of 47 Ω resistors at the inputs, the input noise decreases

to approximately 1.5 nV/√Hz, less than the 4.5 nV/√Hz obtained

when using the internal 500 Ω resistors. However, the −3 dB

bandwidth is reduced from 18 MHz to approximately 3 MHz.

AGC CIRCUIT

The automatic gain control (AGC) circuit compares the rms

output of the part with the desired rms output at the VAGC pin.

Based on this comparison, the DETO pin either sources or sinks

current. By connecting the DETO and GAIN pins together and

by connecting the MODE pin to ground, the AGC circuit can

be used to keep the output rms voltage constant.

To ensure that the AGC circuit reacts fast enough to adjust the

gain, but slow enough to allow signals through, place a capacitor

from DETO to ground. For example, in an on-off keying (OOK)

application with a carrier frequency of 6.795 MHz and a bit rate

of 10 kb/sec, a capacitor value of 0.01 µF is recommended. This

value ensures that the gain reacts to the bit energy but does not

react to the carrier signal.

To set the target rms output voltage, apply a voltage to VAGC.

The target output voltage is lowest when VAGC is set to 1.5 V

and increases when the applied voltage diverges from the 1.5 V

reference voltage. To enable an increasing voltage at the VAGC

pin to increase the rms output voltage, use Equation 7.

(7)

To enable a decreasing voltage at the VAGC pin to increase

the rms output voltage, use Equation 8.

(8)

If the AGC feature is not used, tie the DETO pin to COMM.