±5 V

V

(V±)

Optional

(2)

C1

100 nF

R1

(1)

100

R2

50 k

C2

100 nF

V

(V+)

5 V

SD1

BAS40

V

I

0 V to 4.096 V

ADS8326

16-Bit

250 kSPS

C3

(1)

1 nF

C4

100 nF

5 V

REF IN

±I x

N

+I x

N

REF3240

4.096 V

5 V

OPA2356-EP

R

(S)

100 kΩ

V

(+INx)

I

IB

Device

V

(V+)

V

(V–)

R

(F)

R

(G)

V

O

17

OPA2356-EP

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SBOS955 – FEBRUARY 2019

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8.2.3 Driving ADCs

The OPA2356-EP op amps are designed for driving sampling analog-to-digital converters (ADCs) with sampling

speeds up to 1 MSPS. The zero-crossover distortion input stage topology allows the OPA2356-EP to drive ADCs

without degradation of differential linearity and THD.

The OPA2356-EP can be used to buffer the ADC switched input capacitance and resulting charge injection while

providing signal gain. Figure 34 shows the OPA2356-EP configured to drive the ADS8326.

(1)

Suggested value; may require adjustment based on specific application.

(2)

Single-supply applications lose a small number of ADC codes near ground as a result of op amp output swing

limitation. If a negative power supply is available, this simple circuit creates a –0.3-V supply to allow output swing to

true ground potential.

Figure 34. Driving the ADS8326

8.2.4 Active Filter

The OPA2356-EP is designed for active filter applications that require a wide bandwidth, fast slew rate, low-

noise, single-supply operational amplifier. Figure 35 depicts a 500-kHz, second-order, low-pass filter using the

multiple-feedback (MFB) topology. The components are selected to provide a maximally-flat Butterworth

response. Beyond the cutoff frequency, roll-off is –40 dB/dec. The Butterworth response is preferred for

applications requiring predictable gain characteristics, such as the anti-aliasing filter used in front of an ADC.

One point to observe when considering the MFB filter is that the output is inverted, relative to the input. If this

inversion is not required, or not desired, a noninverting output can be achieved through one of the following

options:

1. Adding an inverting amplifier,

2. Adding an additional second-order MFB stage,

3. Using a noninverting filter topology, such as the Sallen-Key (see Figure 36).