OPA337_16 Datasheet, PDF(9/25 Page) Texas Instruments – MicroSIZE, Single-Supply CMOS OPERATIONAL AMPLIFIERS

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OPA337_16 Datasheet, PDF (9/25 Pages) Texas Instruments – MicroSIZE, Single-Supply CMOS OPERATIONAL AMPLIFIERS

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The required compensation capacitor value can be

determined from the following equation:

C1 = 1/(2ÏfCR1)

Since fC may shift with process variations, it is

recommended that a value less than fC be used for

determining C1. With fC = 1MHz and R1 = 2.5kâ¦, the

compensation capacitor is about 68pF.

The selection of the compensation capacitor C1 is

important. A proper value ensures that the closed-loop

circuit gain is greater than or equal to 5 at high frequencies.

Referring to the Open-Loop Gain vs Frequency plot in the

Typical Characteristics section, the OPA338 gain line

(dashed in the curve) has a constant slope

(â20dB/decade) up to approximately 3MHz. This

frequency is referred to as fC. Beyond fC the slope of the

curve increases, suggesting that closed-loop gains less

than 5 are not appropriate.

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.

Improved slew rate versus OPA337

(see Figure 5).

5kâ¦

VIN

15pF

10kâ¦

150pF

OPA338

VOUT

C2 = 2ÏfCR2 , C1 = (GH â 1) Ã C2

Where GH is the highâfrequency gain,

GH = 1 + C1/C2

Figure 4. Inverting Compensation Circuit of the

OPA338 for Low Gain

Resistors R1 and R2 are chosen to set the desired dc

signal gain. Then the value for C2 is determined as follows:

C2 = 1/(2ÏfCR2)

OPA337, OPA2337

OPA338, OPA2338

SBOS077B â JUNE 1997 â REVISED MARCH 2005

C1 is determined from the desired high-frequency gain (GH):

C1 = (GH â 1) Ã C2

For a desired dc gain of 2 and high-frequency gain of 10,

the following resistor and capacitor values result:

R1 = 10kâ¦

C1 = 150pF

R2 = 5kâ¦

C2 = 15pF

The capacitor values shown are the nearest standard

values. Capacitor values may need to be adjusted slightly

to optimize performance. For more detailed information,

consult the section on Low Gain Compensation in the

OPA846 data sheet (SBOS250) located at www.ti.com.

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.

OPA338

OPA337

Time (2Âµs/div)

Figure 5. G = 2, Slew-Rate Comparison of the

OPA338 and the OPA337

TYPICAL APPLICATION

See Figure 6 for the OPA2337 in a typical application. The

ADS7822 is a 12-bit, micropower, 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, the OPA2337âs 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

data sheet (SBAS062) located at www.ti.com.

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