Datasheet
OPA698
15
SBOS258D
www.ti.com
TYPICAL APPLICATIONS
WIDEBAND VOLTAGE LIMITING OPERATION
The OPA698 is a voltage feedback amplifier that combines
features of a wideband, high slew rate amplifier with output
voltage limiters. Its output can swing up to 1V from each rail
and can deliver up to 120mA. These capabilities make it an
ideal interface to drive ADC while adding overdrive protection
for the ADC inputs.
Figure 1 shows the DC-coupled, gain of +2, dual power-
supply circuit configuration used as the basis of the ±5V
Electrical Characteristics and Typical Characteristics. For
test purposes, the input impedance is set to 50Ω with a
resistor to ground and the output impedance is set to 500Ω.
Voltage swings reported in the specifications are taken
directly at the input and output pins. For the circuit of Figure
1, the total output load will be 500Ω || 804Ω = 308Ω. The
voltage limiting pins are set to ±2V through a voltage divider
network between the +Vs and ground for V
H
, and between –
Vs and ground for V
L
. These limiter voltages are adequately
bypassed with a 0.1µF ceramic capacitor to ground. The
limiter voltages (V
H
and V
L
) and the respective bias currents
(I
VH
and I
VL
) have the polarities shown. One additional
component is included in Figure 1. An additional resistor
(174Ω) is included in series with the noninverting input.
Combined with the 25Ω DC source resistance looking back
towards the signal generator, this gives an input bias current-
canceling resistance that matches the 200Ω source resis-
tance seen at the inverting input (see the DC accuracy and
offset control section). The power-supply bypass for each
supply consists of two capacitors: one electrolytic 2.2µF and
one ceramic 0.1µF. The power-supply bypass capacitors are
shown explicitly in Figures 1 and 2, but will be assumed in the
other figures. An additional 0.01µF power-supply decoupling
capacitor (not shown here) can be included between the
two power-supply pins. In practical PC board layouts, this
optional-added capacitor will typically improve the 2nd
harmonic distortion performance by 3dB to 6dB.
SINGLE-SUPPLY, NONINVERTING AMPLIFIER
Figure 2 shows an AC-coupled, noninverting gain amplifier
for single +5V supply operation. This circuit was used for AC
characterization of the OPA698, with a 50Ω source (which it
matches) and a 500Ω load. The mid-point reference on the
noninverting input is set by two 806Ω resistors. This gives an
input bias current-canceling resistance that matches the
402Ω DC source resistance seen at the inverting input (see
the DC accuracy and offset control section). The power-
supply bypass for the supply consists of two capacitors: one
electrolytic 2.2µF and one ceramic 0.1µF. The power-supply
bypass capacitors are shown explicitly in Figures 1 and 2, but
will be assumed in the other figures. The limiter voltages (V
H
and V
L
) and the respective bias currents (I
VH
and I
VL
) have
the polarities shown. These limiter voltages are adequately
bypassed with a 0.1µF ceramic capacitor to ground. Notice
that the single-supply circuit can use three resistors to set V
H
and V
L
, where the dual-supply circuit usually uses four to
reference the limit voltages to ground. While this circuit
shows +5V operation, the same circuit may be used for
single supplies up to +12V.
OPA698
49.9Ω
6
I
VH
V
O
V
IN
I
VL
–V
S
= –5V
3
2
4
7
8
5
R
F
402Ω
R
G
402Ω
500Ω
0.1µF
0.1µF0.1µF
174Ω
3.01kΩ 1.91kΩ
3.01kΩ 1.91kΩ
0.1µF
V
H
= +2V
V
L
= –2V
+
2.2µF
+
2.2µF
+V
S
= +5V
OPA698
57.6Ω
6
I
VH
V
H
= 3.7V
V
O
V
L
= 1.3V
V
IN
I
VL
806Ω
3
2
4
7
8
5
806Ω
523Ω
976Ω
523Ω
R
G
402Ω
R
F
402Ω
500Ω
0.1µF
0.1µF
0.1µF
+
2.2µF
0.1µF
V
S
= +5V
0.1µF
0.1µF
FIGURE 1. DC-Coupled, Dual-Supply Amplifier.
FIGURE 2. AC-Coupled, Single-Supply Amplifier.