ADA4805-2ACPZ-R2 datasheet(23/25 Pages) AD

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Data Sheet

ADA4805-1/ADA4805-2

Rev. B | Page 23 of 25

SINGLE-ENDED TO DIFFERENTIAL CONVERSION

Most high resolution ADCs have differential inputs to reduce

common-mode noise and harmonic distortion. Therefore, it is

necessary to use an amplifier to convert a single-ended signal

into a differential signal to drive the ADCs.

There are two common ways the user can convert a single-ended

signal into a differential signal: either use a differential

amplifier, or configure two amplifiers as shown in Figure 64.

The use of a differential amplifier yields better performance,

whereas the 2-op-amp solution results in lower system cost. The

ADA4805-1/ADA4805-2 solve this dilemma of choosing between

the two methods by combining the advantages of both. Their

low harmonic distortion, low offset voltage, and low bias current

mean that they can produce a differential output that is well

matched with the performance of the high resolution ADCs.

Figure 64 shows how the ADA4805-1/ADA4805-2 convert a

single-ended signal into a differential output. The first amplifier

is configured in a gain = +1 with its output then inverted to

produce the complementary signal. The differential output then

drives the AD7982, an 18-bit, 1 MSPS SAR ADC. To further

reduce noise, the user can reduce the values of R1 and R2.

However, note that this increases the power consumption. The

low-pass filter of the ADC driver limits the noise to the ADC.

The measured SNR, THD, and SINAD of the whole system for a

10 kHz signal are 93 dB, 113 dBc, and 93 dB, respectively. This

translates to an ENOB of 15.1 at 10 kHz, which is compatible

with the performance of the AD7982. Table 11 shows the

performance of this setup at selected input frequencies.

Table 11. System Performance at Selected Input Frequency

for Driving the AD7982 Differentially

Results

Input Frequency

(kHz)

SNR

(dB)

THD

(dBc)

SINAD

(dB)

ENOB

104

15.1

113

15.1

110

15.1

102

14.8

100

14.3

LAYOUT CONSIDERATIONS

To ensure optimal performance, careful and deliberate attention

must be paid to the board layout, signal routing, power supply

bypassing, and grounding.

Ground Plane

It is important to avoid ground in the areas under and around the

input and output of the ADA4805-1/ADA4805-2. Stray capacitance

between the ground plane and the input and output pads of a

device is detrimental to high speed amplifier performance.

Stray capacitance at the inverting input, together with the

amplifier input capacitance, lowers the phase margin and can

cause instability. Stray capacitance at the output creates a pole in

the feedback loop, which can reduce phase margin and cause

the circuit to become unstable.

Power Supply Bypassing

Power supply bypassing is a critical aspect in the performance

of the ADA4805-1/ADA4805-2. A parallel connection of

capacitors from each power supply pin to ground works best.

Smaller value ceramic capacitors offer better high frequency

response, whereas larger value ceramic capacitors offer better

low frequency performance.

Paralleling different values and sizes of capacitors helps to ensure

that the power supply pins are provided with a low ac impedance

across a wide band of frequencies. This is important for minimizing

the coupling of noise into the amplifier—especially when the

amplifier PSRR begins to roll off—because the bypass capacitors

can help lessen the degradation in PSRR performance.

Place the smallest value capacitor on the same side of the board

as the amplifier and as close as possible to the amplifier power

supply pins. Connect the ground end of the capacitor directly to

the ground plane.

It is recommended that a 0.1 μF ceramic capacitor with a

0508 case size be used. The 0508 case size offers low series

inductance and excellent high frequency performance. Place a

10 μF electrolytic capacitor in parallel with the 0.1 μF capacitor.

Depending on the circuit parameters, some enhancement to

performance can be realized by adding additional capacitors.

Each circuit is different and must be analyzed individually for

optimal performance.

ADA4805-1/

ADA4805-2

ADA4805-1/

ADA4805-2

VIN

+7.5V

+2.5V

VDD

1kΩ

2.7nF

0.1µF

2.7nF

0.1µF

22Ω

AD7982

IN+

IN–

REF

+5V

VDD

Figure 64. Driving the AD7982 with the ADA4805-1/ADA4805-2

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