Figure 4 shows a compensation technique using an inverting
configuration. The low frequency gain is set by the resistor
ratio while the high frequency gain is set by the capacitor
ratio. As with the noninverting circuit, for frequencies above
fC the gain must be greater than the recommended minimum
stable gain for the op amp.
The capacitor values shown are the nearest standard values.
Capacitor values may need to be adjusted slightly to opti-
mize performance. For more detailed information, consult
the OPA686 product data sheet.
Figure 5 shows the large-signal transient response using the
circuit given in Figure 4. As shown, the OPA338 is stable in
low gain applications and provides improved slew rate
performance when compared to the OPA337.
C2
15pF
Improved slew rate versus OPA337
(see Figure 5).
R1
R2
5kΩ
10kΩ
OPA338
VIN
C1
150pF
OPA338
VOUT
OPA337
1
C2 =
, C1 = (GH –1) • C2
2πfCR2
2µs/div
Where GH is the high frequency gain,
GH = 1 + C1/C2
FIGURE 5. G = 2, Slew-Rate Comparison of OPA338 and
OPA337.
FIGURE 4. Inverting Compensation Circuit of OPA338 for
Low Gain.
TYPICAL APPLICATION
Figure 6 shows the OPA2337 in a typical application. The
ADS7822 is a 12-bit, micro-power sampling analog-to-
digital converter available in the tiny MSOP-8 package. As
with the OPA2337, it operates with a supply voltage as
low as +2.7V. When used with the miniature SOT23-8
package of the OPA2337, the circuit is ideal for space-
limited and low power applications. In addition, OPA2337’s
high input impedance allows large value resistors to be
used which results in small physical capacitors, further
reducing circuit size. For further information, consult the
ADS7822 product data sheet.
Resistors R1 and R2 are chosen to set the desired dc signal
gain. Then the value for C2 is determined as follows:
C2 = 1/(2πfCR2)
C1 is determined from the desired high frequency gain (GH):
C1 = (GH – 1) • C2
For a desired dc gain of 2 and high frequency gain of 10, the
following resistor and capacitor values result:
R1 = 10kΩ
R2 = 5kΩ
C1 = 150pF
C2 = 15pF
®
OPA337, OPA2337
OPA338, OPA2338
8
BB [ BURR-BROWN CORPORATION ]
