AD8137YCP-R2 datasheet(20/24 Pages) AD

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AD8137

Rev. B | Page 20 of 24

GND

VREF

VREFA

ADR525A

2.5V SHUNT

REFERENCE

AD7450A

VIN+

VIN–

VDD

AD8137

–

VREFB

2.5V

VOCM

Ω

2.5k

Ω

VIN

1.0nF

0.1

µF

0.1

µF

+1.88V

+1.25V

VACM WITH

VREFB = 0

+0.63V

+2.5V

GND

–2.5V

Figure 64. AD8137 Driving AD7450A, 12-Bit A/D Converter

The input impedance of a conventional inverting op amp

configuration is simply RG, but is higher in Equation 19

because a fraction of the differential output voltage appears at

the summing junctions, VAN and VAP. This voltage partially

bootstraps the voltage across the input resistor RG, leading to

the increased input resistance.

Input Common-Mode Swing Considerations

In some single-ended-to-differential applications when using a

single-supply voltage, attention must be paid to the swing of the

input common-mode voltage, VACM.

Consider the case in Figure 64, where VIN is 5 V p-p swinging

about a baseline at ground and VREFB is connected to ground.

The input signal to the AD8137 is originating from a source

with a very low output resistance.

The circuit has a differential gain of 1.0 and β = 0.5. VICM has an

amplitude of 2.5 V p-p and is swinging about ground. Using the

results in Equation 16, the common-mode voltage at the AD8137’s

inputs, VACM, is a 1.25 V p-p signal swinging about a baseline of

1.25 V. The maximum negative excursion of VACM in this case is

0.63 V, which exceeds the lower input common-mode voltage limit.

One way to avoid the input common-mode swing limitation is

to bias VIN and VREF at midsupply. In this case, VIN is 5 V p-p

swinging about a baseline at 2.5 V, and VREF is connected to a

low-Z 2.5 V source. VICM now has an amplitude of 2.5 V p-p and

is swinging about 2.5 V. Using the results in Equation 17, VACM

is calculated to be equal to VICM because VOCM = VICM. Therefore,

VICM swings from 1.25 V to 3.75 V, which is well within the

input common-mode voltage limits of the AD8137. Another

benefit seen by this example is that since VOCM = VACM = VICM,

no wasted common-mode current flows. Figure 65 illustrates a

way to provide the low-Z bias voltage. For situations that do not

require a precise reference, a simple voltage divider will suffice

to develop the input voltage to the buffer.

VIN

0V TO 5V

AD8137

–

VOCM

Ω

10k

Ω

0.1

µF

0.1

µF

0.1

µF

AD8031

–

0.1

µF

ADR525A

2.5V SHUNT

REFERENCE

AD7450A

VREF

Figure 65. Low-Z Bias Source

Another way to avoid the input common-mode swing limita-

tion is to use dual power supplies on the AD8137. In this case,

the biasing circuitry is not required.

Bandwidth vs. Closed-Loop Gain

The AD8137’s 3 dB bandwidth will decrease proportionally to

increasing closed-loop gain in the same way as a traditional

voltage feedback operational amplifier. For closed-loop gains

greater than 4, the bandwidth obtained for a specific gain can be

estimated as

)

(

MHz

−

(20)

or equivalently, β(72 MHz).

This estimate assumes a minimum 90 ° phase margin for the

amplifier loop, a condition approached for gains greater than

four. Lower gains will show more bandwidth than predicted by

the equation due to the peaking produced by the lower phase

margin.

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