OPA2832 datasheet(21/37 Pages) NSC

21 / 37 page

SINGLE-SUPPLY ACTIVE FILTER

1/2

OPA2832

+5V

−5V

Ω

400

Ω

175

Ω

ΩSource

150

Ω

400

Ω

6.8

µF

6.8

µF

0.1

µF

0.1

µF

0.01

µF

SINGLE-SUPPLY ADC INTERFACE

−3

−6

−9

−12

−15

−18

Frequency (Hz)

100

10k

100k

10M

1/2

OPA2832

400

Ω

400

Ω

2.26k

Ω

374

Ω

22pF

+3.3V

100

Ω

+3.3V

1/2

ADS5203

10−Bit

30MSPS

OPA2832

www.ti.com ............................................................................................................................................. SBOS327C – FEBRUARY 2005 – REVISED AUGUST 2008

The OPA2832, while operating on a single +3.3V or

+5V supply, lends itself well to high-frequency active

filter designs. Again, the key additional requirement is

to establish the DC operating point of the signal near

the supply midpoint for highest dynamic range.

Figure 66 shows an example design of a 1MHz

low-pass Butterworth filter using the Sallen-Key

topology.

Both the input signal and the gain setting resistor are

AC-coupled using 0.1

µF blocking capacitors (actually

giving bandpass response with the low-frequency

pole set to 3.2kHz for the component values shown).

As discussed for Figure 61, this allows the midpoint

bias formed by one 2k

Ω and one 3kΩ resistor to

appear at both the input and output pins. The

midband signal gain is set to +2 (6dB) in this case.

The capacitor to ground on the noninverting input is

intentionally set larger to dominate input parasitic

terms. At a gain of +2, the OPA2832 on a single

supply will show 75MHz small- and large-signal

Figure 63. DC-Coupled, G = +2, Bipolar Supply

bandwidth. The resistor values have been slightly

Specification and Test Circuit

adjusted to account for this limited bandwidth in the

amplifier stage. Tests of this circuit, shown in

Figure 65, illustrate a precise 1MHz, –3dB point with

maximally-flat

passband

(above

the

3.2kHz

The ADC interface in Figure 64 shows a DC-coupled,

AC-coupling corner), and a maximum stop band

single-supply ADC driver circuit. Many systems are

attenuation of 36dB.

now requiring +3.3V supply capability of both the

ADC and its driver. The OPA2832 provides excellent

performance in this demanding application. Its large

input and output voltage ranges and low distortion

support converters such as the ADS5203. The input

level-shifting circuitry was designed so that VIN can

be between 0V and 0.5V, while delivering an output

voltage of 1V to 2V for the ADS5203.

Figure 65. 1MHz, 2nd-Order, Butterworth

Low-Pass Filter

Figure 64. DC-Coupled, +3V ADC Driver

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