OPA690 datasheet(23/45 Pages) TI

23 / 45 page

DC LEVEL-SHIFTING

NG=G+V

R =R /G

R =R /(NG

OUT

(1)

NG=1+R /R

=(G)V

+(NG

G)V

OUT

(2)

1/4

OPA4830

OUT

1/4

OPA4830

+5V

-5V

50W

750W

348W

Source

150W

750W

6.8 F

0.1 F

0.01 F

OPA4830

www.ti.com.................................................................................................................................................... SBOS350A – DECEMBER 2006 – REVISED MAY 2008

Figure 74 illustrates the dc-coupled, gain of +2V/V,

dual power-supply circuit configuration used as the

Figure 75 shows a DC-coupled noninverting amplifier

basis of the ±5V Electrical Characteristics and Typical

that level-shifts the input up to accommodate the

Characteristics.

For

test

purposes,

the

input

desired output voltage range. Given the desired

impedance is set to 50

Ω with a resistor to ground and

signal gain (G), and the amount VOUT needs to be

the output impedance is set to 150

Ω with a series

shifted up (

ΔVOUT) when VIN is at the center of its

output resistor. Voltage swings reported in the

range, Equation 1 and Equation 2 give the resistor

specifications are taken directly at the input and

values

that

produce

the

desired

performance.

output pins. For the circuit of Figure 74, the total

Assume that R4 is between 200Ω and 1.5kΩ.

effective load is 150

Ω

1.5k

Ω. Two optional

components are included in Figure 74. An additional

resistor

(348

Ω) is

included

series

with

the

noninverting input. Combined with the 25

Ω dc source

resistance looking back towards the signal generator,

this gives an input bias current cancelling resistance

that matches the 375

Ω source resistance seen at the

where:

inverting input (see the DC Accuracy and Offset

Control section). In addition to the usual power-supply

decoupling capacitors to ground, a 0.01

µF capacitor

is included between the two power-supply pins. In

Make sure that VIN and VOUT stay within the specified

practical printed circuit board layouts, this optional

input and output voltage ranges.

capacitor

typically

improves

the

2nd-harmonic

distortion performance by 3dB to 6dB.

Figure 75. DC Level-Shifting

The circuit on the front page 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

Figure 74. DC-Coupled, G = +2V/V, Bipolar Supply

and 2V when using a +3V supply. This output means

Specification and Test Circuit

G = 2.00, and

ΔVOUT = 1.50V – G × 0.25V = 1.00V.

Plugging these values into Equation 1 and Equation 2

(with R4 = 750Ω) gives: NG = 2.33, R1 = 375Ω, R2 =

2.25k

Ω, and R3 = 563Ω. The resistors were changed

to the nearest standard values for the front page

circuit.

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