English
Language : 

OPA2680 Datasheet, PDF (14/22 Pages) Burr-Brown (TI) – Dual Wideband, Voltage Feedback OPERATIONAL AMPLIFIER With Disable
◁
HIGH SPEED DELAY CIRCUIT
The OPA2680 makes an ideal amplifier for a variety of
active filter designs. Shown in Figure 7 is a circuit that
utilizes the two amplifiers within the dual OPA2680 to
design a two-stage analog group delay adjustment circuit.
For simplicity, the circuit assumes a dual supply (±5V)
operation, but it can also be modified to operate on signal
supply. The input to the first filter stage is driven by the
wideband buffer amplifier, BUF601 to isolate the signal
input from the filter network. The BUF601 features an open-
loop design, holding its output impedance at 4Ω for frequen-
cies in excess of 200MHz.
Each of the two filter stages is a 1st-order filter with a
voltage gain of +1. The delay time through one filter is given
by Equation 1.
Equation 1:
tGR0 = 2RC
For a more accurate analysis of the circuit consider the group
delay for the amplifiers. For example, in the case of the
OPA2680, the group delay in the bandwidth from 1MHz to
100MHz is approximately 1.0ns. To account for this, modify
the transfer function, which now comes out to be:
Equation 2:
tGR = 2 (2RC +TD)
with TD = (1/360) • (dφ/df) = delay of the op amp itself. The
values of resistors RF and RG should be equal and low to
avoid parasitic effects. If the all-pass filter is designed for
very low delay times, include parasitic board capacitances
to calculate the correct delay time. Simulating this applica-
tion using the PSPICE model of the OPA2680 will allow this
design to be tuned.
DIFFERENTIAL RECEIVER/DRIVER
A very versatile application for a dual operational amplier
is the differential amplifier configuration shown in Figure
8. With both amplifiers of the OPA2680 connected for
non-inverting operation, the circuit provides a high input
impedance while the gain can easily be set by just one
resistor, RG. When operated in low gains, the output swing
may be limited as a result of the common-mode input
swing limits of the amplifier itself. An interesting modifi-
cation of this circuit is to place a capacitor in series with
the RG. Now the DC gain for each side is reduced to +1,
while the AC gain still follows the standard transfer func-
tion of G = 1 + 2RF/RG. This might be advantageous for
applications processing only a frequency band which ex-
cludes DC or very low frequencies. An input DC voltage
resulting from input bias currents will not be amplified by
the AC gain and can be kept low. This circuit can be used
as a differential line receiver, driver, or as an interface to
a differential input A/D converter.
VIN
BUF601
C
R
1/2
OPA2680
RG
402Ω
RF
402Ω
C
R
RG
402Ω
1/2
OPA2680
RF
402Ω
VOUT
FIGURE 7. Two Stage, All-Pass Network.
50Ω
VI
1/2
RO
OPA2680
RF
402Ω
RG
RF
402Ω
1/2
RO
50Ω
OPA2680
VI
FIGURE 8. High Speed Differential Receiver.
®
OPA2680
14
VDIFF = 1 +
2RF
RG
VI – VI
 ▷