OPA634U datasheet(12/18 Pages) TI1

12 / 18 page

OPA634, OPA635

SBOS097A

DC LEVEL SHIFTING

Figure 3 shows a DC-coupled non-inverting amplifier that

level-shifts the input up to accommodate the desired output

voltage range. Given the desired signal gain (G), and the

amount VOUT needs to be shifted up (∆VOUT) when VIN is at

the center of its range, the following equations give the

resistor values that produce the desired performance. Start

by setting R4 between 200Ω and 1.5kΩ.

NG = G +

∆V

OUT/VS

R1 = R4/G

R2 = R4/(NG – G)

R3 = R4/(NG –1)

where:

NG = 1 + R4/R3

VOUT = (G)VIN + (NG – G)VS

Make sure that VIN and VOUT stay within the specified input

and output voltage ranges.

NON-INVERTING AMPLIFIER WITH

REDUCED PEAKING

Figure 4 shows a non-inverting amplifier that reduces peak-

ing at low gains. The resistor RC compensates the OPA634

or OPA635 to have higher Noise Gain (NG), which reduces

the AC response peaking (typically 5dB at G = +1 without

RC) without changing the DC gain. VIN needs to be a low

impedance source, such as an op amp. The resistor values

are low to reduce noise. Using both RT and RF helps

minimize the impact of parasitic impedances.

OPA63x

OUT

FIGURE 3. DC Level-Shifting Circuit.

OPA63x

OUT

FIGURE 4. Compensated Non-Inverting Amplifier.

The front page circuit is a good example of this type

of application. It was designed to take VIN between

0V and 0.5V, and produce VOUT between 1V and 2V,

when using a +3V supply. This means G = 2.00, and

∆V

OUT = 1.50V – G • 0.25V = 1.00V. Plugging into the

above equations (with R4 = 750Ω) gives: NG = 2.33,

R1 = 375Ω, R2 = 2.25kΩ, and R3 = 563Ω. The resistors

were changed to the nearest standard values.

The Noise Gain can be calculated as follows:

G G

A unity gain buffer can be designed by selecting RT = RF =

20.0

Ω and R

C = 40.2Ω (do not use RG). This gives a Noise

Gain of 2, so its response will be similar to the Characteris-

tics Plots with G = +2. Decreasing RC to 20.0Ω will increase

the Noise Gain to 3, which typically gives a flat frequency

response, but with less bandwidth.

The circuit in Figure 1 can be redesigned to have less

peaking by increasing the noise gain to 3. This is accom-

plished by adding RC = 2.55kΩ between the op amps inputs.

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