OPA683IDBVT datasheet(14/31 Pages) TI

14 / 31 page

OPA683

SBOS221E

www.ti.com

VERY LOW POWER ACTIVE FILTER

The OPA683 provides an exceptionally capable gain block

for implementing Sallen-Key type filters. Typically, the band-

width interaction with gain setting for low power amplifiers,

constrain these filters to using unity-gain amplifiers. Since

the OPA683 CFBplus design holds very high bandwidth to

high gains, implementations that provide signal gain, as well

as the desired filter shape, are easily implemented. Figure 6

shows an example of a 5MHz 2nd-order low-pass filter where

the amplifier is providing a voltage gain of 4. This single-

supply implementation (applicable to single +12V operation

as well) consumes only 5.1mW quiescent power. The two

12.5k

Ω resistors bias the input and output at the supply

midpoint while the three 0.1

µF capacitors block off the DC

current paths to ground for this mid-scale operating point.

The filter resistors and capacitors have been adjusted

to provide a Butterworth (Q = 0.707) response with a

O = 2π • 5MHz. This gives a flat passband response with a

–3dB cutoff at 5MHz. Figure 7 shows the small-signal fre-

quency response for the circuit of Figure 6.

HIGH GAIN HF AMPLIFIER

Where high gains at moderate frequencies are required in an

HF receiver channel, the OPA683 can provide a very low

power solution with moderate input noise figure. Figure 8

shows a technique that can improve the noise figure with no

added power. An input transformer provides a noiseless

voltage gain at the cost of higher source impedance for the

amplifier’s noninverting input current noise. The circuit of

Figure 8, using a 1:4 turns ratio (1:16 impedance ratio)

transformer, reduces the input noise figure from about 20dB

for just the amplifier to 10.6dB in combination. The bandwidth

for this circuit will be principally set by the transformer since

the OPA683 will give > 80MHz for the gain of 20V/V shown

in Figure 8. The overall circuit gives a gain to a matched 50

Ω

load of 32dB (40V/V) from the transformer input. This ex-

ample circuit provides this gain using only 10mW of quies-

cent power with application from 500kHz to 30MHz.

1.4k

Ω

OPA683

+5V

12.5k

Ω

12.5k

Ω

467

Ω

0.1

µF

0.1

µF

446

Ω

157

Ω

100pF

Supply

De-coupling

Not Shown

Ω

150pF

0.1

µF

–3

–6

–9

Frequency (Hz)

20M

100k

10k

10M

LOW POWER 5MHz LP ACTIVE FILTER

FIGURE 6. 5MHz, 2nd-Order Low Pass Filter.

FIGURE 7. Low Power Active Filter Frequency Response.

OPA683

+5V

–5V

Ω

0.01

µF

800

Ω

O = 32dB

1.2k

Ω

10.6dB

Noise Figure

1:4

FIGURE 8. Low Power, High Gain HF Amplifier.

LOW POWER, ADC DRIVER

Where a low power, single-supply interface to a single-ended

input +5V ADC is required, the circuit of Figure 9 can provide

a very flexible, high performance solution. Running in an AC-

coupled inverting mode allows the noninverting input to be

used for the common-mode voltage from the ADS820 con-

verter. This midpoint reference biases both the noninverting

converter input and the amplifier noninverting input. With an

AC-coupled gain path, this +2.5V DC bias has a gain of +1

to the output putting the output at the DC midpoint for the

converter. The output then drives through an isolating resis-

tor (60

Ω) to the inverting input of the converter which is

further decoupled by a 22pF external capacitance to add to

its 5pF input capacitance. This coupling network provides a

high cutoff low-pass while also giving a low source imped-

ance at high frequencies for the converter. The gain for this

circuit is set by adjusting RG to the desired value. For a 2VPP

maximum output driving the light load of Figure 9, the

OPA683 will provide < –80dBc THD through 1MHz as shown

in the Typical Characteristics. One of the important advan-

tages for this CFBplus amplifier is that this distortion does not

degrade significantly at higher gains.

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