LTC6409

16

6409fa

applicaTions inForMaTion

between the R

L

• 4 = 200Ω differential resistance seen at

location B and the 200Ω formed by the two 100Ω match-

ing resistors at the LTC6409 output. Thus, the differential

power at location B is 10 – 6 = 4dBm. Since the transformer

ratio is 4:1 and it has an insertion loss of about 1dB, the

power at location C (across R

L

) is calculated to be 4 – 6

– 1 = –3dBm. This means that IMD3 should be measured

while the power at the output of the demo board is –3dBm

which is equivalent to having 2V

P-P

differential peak (or

10dBm) at the output of the LTC6409.

GBW vs f

–3dB

Gain-bandwidth product (GBW) and –3dB frequency (f

–3dB

have been both specified in the Electrical Characteristics

table as two different metrics for the speed of the LTC6409.

GBW is obtained by measuring the gain of the amplifier

at a specific frequency (f

TEST

) and calculate gain • f

TEST

To measure gain, the feedback factor (i.e.

b = R

I

/(R

I

+

R

F

)) is chosen sufficiently small so that the feedback loop

does not limit the available gain of the LTC6409 at f

TEST

ensuring that the measured gain is the open loop gain of

the amplifier. As long as this condition is met, GBW is a

parameter that depends only on the internal design and

compensation of the amplifier and is a suitable metric to

specify the inherent speed capability of the amplifier.
f

–3dB

, on the other hand, is a parameter of more practi-

cal interest in different applications and is by definition

the frequency at which the gain is 3dB lower than its low

frequency value. The value of f

–3dB

depends on the speed

of the amplifier as well as the feedback factor. Since the

LTC6409 is designed to be stable in a differential signal

gain of 1 (where R

I

= R

F

or

b = 1/2), the maximum f

–3dB

is obtained and measured in this gain setting, as reported

in the Electrical Characteristics table.
In most amplifiers, the open loop gain response exhibits a

conventional single-pole roll-off for most of the frequen-

cies before crossover frequency and the GBW and f

–3dB

numbers are close to each other. However, the LTC6409 is

intentionally compensated in such a way that its GBW is

significantly larger than its f

–3dB

. This means that at lower

frequencies (where the input signal frequencies typically lie,

e.g. 100MHz) the amplifier’s gain and the thus the feedback

loop gain is larger. This has the important advantage of

further linearizing the amplifier and improving distortion

at those frequencies.
Looking at the Frequency Response vs Closed Loop Gain

graph in the Typical Performance Characteristics section

of this data sheet, one sees that for a closed loop gain

(A

V

) of 1 (where R

I

= R

F

= 150Ω), f

–3dB

However, for A

V

= 400 (where R

I

= 25Ω and R

F

= 10kΩ),

the gain at 100MHz is close to 40dB = 100V/V, implying

a GBW value of 10GHz.

Feedback Capacitors
When the LTC6409 is configured in low differential gains,

it is often advantageous to utilize a feedback capacitor (C

F

in parallel with each feedback resistor (R

F

). The use of C

F

implements a pole-zero pair (in which the zero frequency

is usually smaller than the pole frequency) and adds posi-

tive phase to the feedback loop gain around the amplifier.

Therefore, if properly chosen, the addition of C

F

boosts

the phase margin and improves the stability response of

the feedback loop. For example, with R

I

= R

F

= 150Ω, it is

recommended for most general applications to use C

F

=

1.3pF across each R

F

. This value has been selected to

maximize f

–3dB

for the LTC6409 while keeping the peaking

of the closed loop gain versus frequency response under

a reasonable level (<1dB). It also results in the highest

frequency for 0.1dB gain flatness (f

0.1dB

).

However, other values of C

F

can also be utilized and tailored

to other specific applications. In general, a larger value

for C

F

reduces the peaking (overshoot) of the amplifier in

both frequency and time domains, but also decreases the

closed loop bandwidth (f

–3dB

). For example, while for a

closed loop gain (A

V

) of 5, C

F

= 0.8pF results in maximum

f

–3dB

(as previously shown in the Frequency Response vs

Closed Loop Gain graph of this data sheet), if C

F

= 1.2pF

is used, the amplifier exhibits no overshoot in the time

domain which is desirable in certain applications. Both the

circuits discussed in this section have been shown in the

Typical Applications section of this data sheet.

DC1591A Datasheet Related Products:
DC1591A Information:
Part No.
DC1591A
Description