OPA678AP BURR-BROWN [Burr-Brown Corporation], OPA678AP Datasheet - Page 7

• Manufacturer Part Number: OPA678AP
• Description: Wideband Switched-Input OPERATIONAL AMPLIFIER
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

THEORY OF OPERATION
The simplified circuit of the ECL compatible OPA678 is
shown in Figure 1. It is a “classical” high-speed op amp
architecture with one important exception—the amplifier
has two ECL logic selectable differential input stages. An
appropriate differential ECL logic signal on A and A will
turn on either Q5 or Q6, steering operating (tail) current to
either differential input pair Q1 and Q2 or Q3 and Q4. The
input pair receiving the tail current operates as a conven-
tional op-amp input stage while the de-selected input pair
receiving no tail current appears as an open circuit. The de-
selected inputs have only a few pF parasitic capacitance and
in the off condition exhibit only a very low leakage (bias)
current of about 100pA. Two feedback networks can be
connected to each input separately allowing a wide range of
circuit applications. The feedback network connected to the
selected input operates in a normal op amp fashion while the
feedback network connected to the de-selected input is
totally inactive, appearing only as an additional load to the
amplifier’s output stage.
For TTL operation, “A select” is held to an internal refer-
ence level by tying pins 13 and 14 together. This allows “A”
to become the single-ended TTL input.
FIGURE 1. OPA678 Simplified Circuit Diagram.
Select
A
Q7
Q8
Select
–
A
Q1
Q5
A
Q2
+ –
–V
I
TAIL
CC
+V
CC
Q3
7
Q6
B
Q4
Standard TTL and ECL logic levels may be applied to each
input selection circuit but only 350mV is typically required
to switch between inputs. This logic input sensitivity allows
simpler high-speed logic driver circuitry and it minimizes
digital noise coupling into adjacent wideband analog cir-
cuitry and allows single ended ECL inputs to be used with
V
The OPA678 is designed to be frequency compensated by a
single capacitor connected from pin 5 to ground. Recom-
mended compensation is shown in the Typical Performance
Curve section. A small variable capacitor may be trimmed
for best bandwidth, settling time, and gain peaking. Closed-
loop gain/phase (Bode) plots are shown in the Typical
Performance Curves.
OFFSET TRIM
The laser trimmed input offset voltage is low enough for
many video and RF applications. Independent control of
input offset will require that trim adjust current be summed
into one or both inputs.
BB
Bias
applied to the other input.
+
Comp
OPA678
Out
®