AD8330 Analog Devices, AD8330 Datasheet - Page 21

• Manufacturer Part Number: AD8330
• Description: Low Cost DC-150MHz Variable Gain Amplifier
• Manufacturer: Analog Devices

Figure 19 shows typical results for V
input amplitude of 450 mV (the actual combination is not impor-
tant) and a rise time of 2 ns. V
upper waveforms the load capacitors are both zero, and a small
amount of overshoot is visible; with 40 pF the response is cleaner.
A shunt capacitance of 20 pF from OPHI to OPLO will have a
similar effect. Coupling capacitors for this demonstration are
sufficiently large to prevent any visible droop over this time scale.
The outputs at the load side will eventually assume a mean value
of zero, with negative and positive excursions depending on the
duty-cycle.
The bandwidth from pin VMAG to these outputs is somewhat
higher than that from the normal input pins. Thus when this pin
is used to rapidly modulate the primary signal, some further
experimentation with response optimization may be required. In
general, the AD8330 is very tolerant of a wide range of loading
conditions.
Preserving Absolute Gain
Although the AD8330 is not laser-trimmed, its absolute gain cali-
bration, being based mainly on ratios, is very good. Full details can
be found in the Specifications and in the typical performance
curves. Nevertheless, having finite input and output impedances,
the gain is necessarily dependent on the source and load condi-
tions. The loss incurred when either of these is finite causes an
error in the absolute gain, which may also be uncertain due to
the approximately 20% tolerance in the absolute value of the
input and output impedances.
Often, such losses and uncertainties can be tolerated and accom-
modated by a correction to the gain control bias. On the other
hand, the error in the loss can be essentially nulled by using
appropriate modifications to either the source impedance (R
or the load impedance (R
them with series or shunt components.
The formulation for this correction technique was described
previously. However, to simplify its use, Table I is provided, showing
spot values for combinations of R
loss that will not be dependent on sample-to-sample variations
in on-chip resistances. Furthermore, this fixed and predictable loss
can be corrected by an adjustment to V
REV. A
Figure 19. Typical Pulse Responses for Figure 18
–0.2
–0.2
–0.4
–0.6
–0.8
–1.0
–1.2
–0.2
–0.2
–0.4
–0.6
–0.8
–1.0
–1.2
1.0
0.8
0.6
0.4
1.2
1.0
0.8
0.6
0.4
0.2
1.2
0.2
0.2
0.2
0
0
0
0
0
5ns
L
10ns
), or both, in some cases by padding
MAG
raised to 2.0 V is used. In the
S
and R
15ns
DBS
MAG
= 0.24 V, a square wave
, as indicated in Table 1.
L
resulting in an overall
20ns
25ns
S
)
–21–
Calculation of Noise Figure
The AD8330 noise is a consequence of its intrinsic voltage-noise-
spectral-density (E
(I
which is a function of the device’s input resistance, R
1 k , and the differential source resistance, R
Note that we assume purely resistive source and input impedances
as a concession to simplicity. A more thorough treatment of noise
mechanisms, for the case where the source is reactive, is beyond the
scope of these brief notes. Also note that V
noise-spectral-density appearing across the differential input pins,
INHI, INLO. In preparing for the calculation of noise figure,
we will define V
source and V
relationship is simply
At maximum gain, E
Thus, the short-circuit voltage noise is:
Next, examine the net noise when R
called the “matching” condition, rather than “source impedance
termination,” which is the actual situation in this case. Repeating
the procedure:
V
R
NSD
NOISE_IN
1.5k
100
150
200
300
500
750
S
10
15
20
30
50
75
1k
2k
V
( )
V
V
NOISE_IN
). Their combined effect generates a net input noise, V
NOISE_IN
IN
7 3 .
5 08
.
R
nV
7.5k
5.0k
3.0k
2.0k
1.5k
1.0k
750
500
300
200
150
100
15k
10k
L
75
R
IN
V
( )
nV
I
Table I. Preserving Absolute Gain
SIG
/
as the differential input to the AD8330. The
Uncorrected LossV
SIG
/
4 1
R
Hz
R
.
NSD
I
as the open-circuit signal voltage across the
S
Hz
4 1
nV
E
NSD
Factor
.
) and the current-noise-spectral-density
0.980
0.971
0.961
0.943
0.907
0.865
0.826
0.756
0.694
0.592
0.444
0.327
0.250
0.160
0.111
NSD
nV
/
is 4.1 nV/ Hz, and I
2
Hz
/
I
Hz
2
NSD
2
S
2
3
= R
0.17
0.26
0.34
0.51
0.85
1.26
1.66
2.43
3.17
4.56
7.04
9.72
12.0
15.9
19.1
dB
R
MAG
pA
I
3
I
pA
= 1 k , often incorrectly
/
Required
R
NOISE_IN
Hz
/
S
to Correct Loss
2
NSD
S
Hz
, as follows:
2
AD8330
1
is 3 pA/ Hz.
2
is the voltage-
k
0.510
0.515
0.520
0.530
0.551
0.578
0.605
0.661
0.720
0.845
1.125
1.531
2.000
3.125
4.500
1
I
, nominally
k
NOISE_IN
1
k
0
(16)
(17)
(18)
(19)
2
2
,