Datasheet

OPA698
22
SBOS258D
www.ti.com
FIGURE 22. Driving Capacitive Loads.
OPA698
C
L
R
L
R
T
R
S
R
G
R
F
V
O
R
L
is optional
THERMAL CONSIDERATIONS
The OPA698 will not require heat sinking under most oper-
ating conditions. Maximum desired junction temperature will
set a maximum allowed internal power dissipation as de-
scribed below. In no case should the maximum junction
temperature be allowed to exceed 150°C.
The total internal power dissipation (P
D
) is the sum of
quiescent power (P
DQ
) and the additional power dissipated in
the output stage (P
DL
) while delivering load power. P
DQ
is
simply the specified no-load supply current times the total
supply voltage across the part. P
DL
depends on the required
output signals and loads. For a grounded resistive load, and
equal bipolar supplies, it is at maximum when the output is
at 1/2 either supply voltage. In this condition, P
DL
= V
S
2
/(4R
L
)
where R
L
includes the feedback network loading. Note that it
is the power in the output stage, and not in the load, that
determines internal power dissipation.
The operating junction temperature is: T
J
= T
A
+ P
D
x θ
JA
,
where T
A
is the ambient temperature. For example, the
maximum T
J
for a OPA698ID with G = +2, R
F
= 402Ω, R
L
=
100Ω, and ±V
S
= ±5V at the maximum T
A
= +85°C is
calculated as:
P V mA mW
P
V
mW
PmWmWmW
TCmWCWC
DQ
DL
D
J
=×
( )
=
=
(
)
×
( )
=
=+=
=°+ × ° = °
10 15 5 155
5
4 100 804
70
155 70 225
85 225 125 113
2
.
||
/
ΩΩ
This would be the maximum T
J
from V
O
= ±2.5V
DC
. Most
applications will be at a lower output stage power and have
a lower T
J
.
CAPACITIVE LOADS
Capacitive loads, such as the input to ADCs, will decrease
the amplifier phase margin, which may cause high-frequency
peaking or oscillations. Capacitive loads ≥ 2pF should be
isolated by connecting a small resistor in series with the
output, as shown in Figure 22. Increasing the gain from +2
will improve the capacitive drive capabilities due to increased
phase margin.
In general, capacitive loads should be minimized for optimum
high-frequency performance. The capacitance of coax cable
(29pF/ft for RG-58) will not load the amplifier when the
coaxial cable, or transmission line, is terminated in its char-
acteristic impedance.
FREQUENCY RESPONSE COMPENSATION
The OPA698 is internally compensated to be unity-gain
stable, and has a nominal phase margin of 60° at a gain of
+2. Phase margin and peaking improve at higher gains.
Recall that an inverting gain of –1 is equivalent to a gain of
+2 for bandwidth purposes (that is, noise gain = 2). Standard
external compensation techniques work with this device.
For example, in the inverting configuration, the bandwidth
may be limited without modifying the inverting gain by placing
a series RC network to ground on the inverting node. This
has the effect of increasing the noise gain at high frequen-
cies, which limits the bandwidth.
To maintain a wide bandwidth at high gains, cascade several
op amps, or use the high-gain optimized OPA699.
In applications where a large feedback resistor is required,
such as photodiode transimpedance amplifier, the parasitic
capacitance from the inverting input to ground causes peak-
ing or oscillations. To compensate for this effect, connect a
small capacitor in parallel with the feedback resistor. The
bandwidth will be limited by the pole that the feedback
resistor and this capacitor create. In other high-gain applica-
tions, use a three-resistor
Tee network to reduce the RC time
constants set by the parasitic capacitances. Be careful not to
increase the noise generated by this feedback network too
much.
PULSE SETTLING TIME
The OPA698 is capable of an extremely fast settling time in
response to a pulse input. Frequency response flatness and
phase linearity are needed to obtain the best settling times.
For capacitive loads, such as an ADC, use the recom-
mended R
S
in the typical performance curve R
S
vs Capaci-
tive Load
. Extremely fine-scale settling (0.01%) requires
close attention to ground return current in the supply
decoupling capacitors.
The pulse settling characteristics, when recovering from
overdrive, are very good.
DISTORTION
The OPA698 distortion performance is specified for a 500Ω
load, such as an ADC. Driving loads with smaller resistance
will increase the distortion, as illustrated in Figure 23. Re-
member to include the feedback network in the load resis-
tance calculations.