OPA627_07 Datasheet, PDF (9/16 Pages) Texas Instruments – Precision High-Speed Difet OPERATIONAL AMPLIFIERS

OFFSET VOLTAGE ADJUSTMENT
The OPA627/637 is laser-trimmed for low offset voltage
and drift, so many circuits will not require external adjust-
ment. Figure 3 shows the optional connection of an external
potentiometer to adjust offset voltage. This adjustment should
not be used to compensate for offsets created elsewhere in a
system (such as in later amplification stages or in an A/D
converter) because this could introduce excessive tempera-
ture drift. Generally, the offset drift will change by approxi-
mately 4µV/°C for 1mV of change in the offset voltage due
to an offset adjustment (as shown on Figure 3).
C₂
C₁
R₂
–
+ _OPA637
R₁
C₁ = C_IN + C_STRAY
C₂ = ^{R1 C1}
R₂
FIGURE 2. Circuits with Noise Gain Equal to or Greater than
Five May Use the OPA637.
NOISE PERFORMANCE
Some bipolar op amps may provide lower voltage noise
performance, but both voltage noise and bias current noise
contribute to the total noise of a system. The OPA627/637
is unique in providing very low voltage noise and very low
current noise. This provides optimum noise performance
over a wide range of sources, including reactive source
impedances. This can be seen in the performance curve
showing the noise of a source resistor combined with the
noise of an OPA627. Above a 2kΩ source resistance, the op
amp contributes little additional noise. Below 1kΩ, op amp
noise dominates over the resistor noise, but compares
favorably with precision bipolar op amps.
CIRCUIT LAYOUT
As with any high speed, wide bandwidth circuit, careful
layout will ensure best performance. Make short, direct
interconnections and avoid stray wiring capacitance—espe-
cially at the input pins and feedback circuitry.
The case (TO-99 metal package only) is internally connected
to the negative power supply as it is with most common op
amps. Pin 8 of the plastic DIP, SOIC, and TO-99 packages
has no internal connection.
Power supply connections should be bypassed with good
high frequency capacitors positioned close to the op amp
pins. In most cases 0.1µF ceramic capacitors are adequate.
The OPA627/637 is capable of high output current (in
excess of 45mA). Applications with low impedance loads or
capacitive loads with fast transient signals demand large
currents from the power supplies. Larger bypass capacitors
such as 1µF solid tantalum capacitors may improve dynamic
performance in these applications.
+V_S
100kΩ
7
1
2
–
5
10kΩ to 1MΩ
Potentiometer
(100kΩ preferred)
3
6
+
OPA627/637
4
±10mV Typical
Trim Range
–V_S
FIGURE 3. Optional Offset Voltage Trim Circuit.
Non-inverting
Buffer
2–
2–
6
Out
6
Out
In
3₊
In
3₊
OPA627
OPA627
In
Board Layout for Input Guarding:
Guard top and bottom of board.
Alternate—use Teflon^® standoff for sen-
sitive input pins.
Teflon^® E.I. du Pont de Nemours & Co.
Inverting
OPA627
2–
6
Out
3₊
FIGURE 4. Connection of Input Guard for Lowest I_B.
9
TO-99 Bottom View
3
4
5
2
6
7
1
8 No Internal Connection
To Guard Drive
®
OPA627, 637