AD8137YCPZ-R2 datasheet(28/33 Pages) AD

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Data Sheet

AD8137

Rev. E | Page 27 of 32

Figure 67 shows the AD8137 in a unity-gain configuration, and

with the following discussion, provides a good example of how

to provide a proper termination in a 50 Ω environment.

AD8137

–

2V p-p

52.3

Ω

–

VOCM

Ω

1.02k

Ω

0.1

µF

0.1

µF

+5V

–5V

VIN

SIGNAL

SOURCE

Ω

Figure 67. AD8137 with Terminated Input

The 52.3 Ω termination resistor, RT, in parallel with the 1 kΩ

input resistance of the AD8137 circuit, yields an overall input

resistance of 50 Ω that is seen by the signal source. To have

matched feedback loops, each loop must have the same RG if it

has the same RF. In the input (upper) loop, RG is equal to the 1 kΩ

resistor in series with the (+) input plus the parallel combination

of RT and the source resistance of 50 Ω. In the upper loop, RG is

therefore equal to 1.03 kΩ. The closest standard value is 1.02 kΩ

and is used for RG in the lower loop.

Things become more complicated when it comes to determining

the feedback resistor values. The amplitude of the signal source

generator VIN is two times the amplitude of its output signal when

terminated in 50 Ω. Therefore, a 2 V p-p terminated amplitude

is produced by a 4 V p-p amplitude from VS. The Thevenin

equivalent circuit of the signal source and RT must be used when

calculating the closed-loop gain because RG in the upper loop is

split between the 1 kΩ resistor and the Thevenin resistance

looking back toward the source. The Thevenin voltage of the

signal source is greater than the signal source output voltage

when terminated in 50 Ω because RT must always be greater

than 50 Ω. In this case, RT is 52.3 Ω and the Thevenin voltage

and resistance are 2.04 V p-p and 25.6 Ω, respectively.

Now the upper input branch can be viewed as a 2.04 V p-p

source in series with 1.03 kΩ. Because this is to be a unity-gain

application, a 2 V p-p differential output is required, and RF

must therefore be 1.03 kΩ × (2/2.04) = 1.01 kΩ ≈ 1 kΩ.

This example shows that when RF and RG are large compared to RT,

the gain reduction produced by the increase in RG is essentially

cancelled by the increase in the Thevenin voltage caused by RT

being greater than the output resistance of the signal source. In

general, as RF and RG become smaller in terminated applications,

RF needs to be increased to compensate for the increase in RG.

When generating the typical performance characteristics data,

the measurements were calibrated to take the effects of the

terminations on closed-loop gain into account.

Power-Down

The AD8137 features a PD pin that can be used to minimize the

quiescent current consumed when the device is not being used.

PD is asserted by applying a low logic level to Pin 7. The threshold

between high and low logic levels is nominally 1.1 V above the

negative supply rail. See Table 1 to Table 3 for the threshold limits.

The AD8137 PD pin features an internal pull-up network that

enables the amplifier for normal operation. The AD8137 PD

pin can be left floating (that is, no external connection is

required) and does not require an external pull-up resistor to

ensure normal on operation (see Figure 68).

Do not connect the PD pin directly to VS+ in ±5 V applications.

This can cause the amplifier to draw excessive supply current

(see Figure 59) and may induce oscillations and/or stability

issues.

50kΩ

5kΩ

150kΩ

REF A

–VS

+VS

Figure 68. PD Pin Circuit

DRIVING AN ADC WITH GREATER THAN 12-BIT

PERFORMANCE

Because the AD8137 is suitable for 12-bit systems, it is desirable

to measure the performance of the amplifier in a system with

greater than 12-bit linearity. In particular, the effective number

of bits (ENOB) is most interesting. The AD7687, 16-bit, 250 KSPS

ADC performance makes it an ideal candidate for showcasing

the 12-bit performance of the AD8137.

For this application, the AD8137 is set in a gain of 2 and driven

single-ended through a 20 kHz band-pass filter, while the output

is taken differentially to the input of the AD7687 (see Figure 69).

This circuit has mismatched RG impedances and, therefore, has a

dc offset at the differential output. It is included as a test circuit to

illustrate the performance of the AD8137. Actual application

circuits should have matched feedback networks.

For an AD7687 input range up to −1.82 dBFS, the AD8137 power

supply is a single 5 V applied to VS+ with VS− tied to ground. To

increase the AD7687 input range to −0.45 dBFS, the AD8137

supplies are increased to +6 V and −1 V. In both cases, the VOCM

pin is biased with 2.5 V and the PD pin is left floating. All voltage

supplies are decoupled with 0.1 µF capacitors. Figure 70 and

Figure 71 show the performance of the −1.82 dBFS setup and the

−0.45 dBFS setup, respectively.

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