ADA4627-1

Rev. A | Page 13 of 16

oscillator is connected to the circuit under test, and the output

of the circuit goes back to the analyzer.

The analyzer has a tunable notch filter in lock step with the

swept oscillator. This removes the fundamental frequency,

but allows all of the harmonics and wideband noise to be

measured with an integrating voltmeter. However, there is

a switchable low-pass filter in series with the notch filter.

If the sine wave is at 100 Hz, then the tenth harmonic is still

at 1 kHz, thus having a low pass at 80 kHz is not a problem.

When the oscillator reaches 20 kHz, the fourth harmonic

(80 kHz) is partially attenuated, resulting in a lower reading

from the voltmeter. When evaluating THD + N curves from

any manufacturer, careful attention should be paid to the test

conditions. The difference between an 80 kHz low-pass filter

and a 500 kHz filter is shown in Figure 39.

0.01

0.1

1

10

100

FREQUENCY (kHz)

80kHz FILTER

500kHz FILTER

0.01

0.001

0.0001

0.00001

ADA4627-1

Figure 39. THD + N vs. Frequency

PRINTED CIRCUIT BOARD LAYOUT, BIAS

CURRENT, AND BYPASSING

To take advantage of the very low input bias current of the

ADA4627-1 at room temperature, leakage paths must be

considered. A printed circuit board, with dust and humidity,

can have 100 MΩ of resistance over a few tenths of an inch.

A one mV differential between the two points results in 10 pA

of leakage current, more than the guaranteed maximum.

The op amp inputs should be guarded by surrounding the nets

with a metal trace maintained at the predicted voltage. In the

case of an inverting configuration or transimpedance amplifier,

(see Figure 40), the inverting and noninverting nodes can be

surrounded by traces held at a quiet analog ground.

2

3

6

8

ADA4627-1

+

–

GUARD

Figure 40. Inverting Amplifier with Guard

For a noninverting configuration, the trace can be driven from

the feedback divider, but the resistors should be chosen to offer

a low impedance drive to the trace (see Figure 41).

3

2

6

8

ADA4627-1

+

–

+

–

GUARD

Figure 41. Noninverting Amplifier with Guard

The board layout should be compact with traces as short as

possible. For second-order board considerations, such as

triboelectric effects and piezoelectric effects, as well as a table

of insulating material properties, see the AD549 data sheet.

In some cases, shielding from air currents, may be helpful.

A general rule of thumb, for op amps with gain bandwidth

products higher than 1 MHz, bypass capacitors should be very

close to the part, within 3 millimeters. Each supply should be

bypassed with a 0.01 μF ceramic capacitor in parallel with a

1 μF bulk decoupling capacitor. The ceramic capacitors should

be closer to the op amp. Sockets, which add inductance and

capacitance, should not be used.

OUTPUT PHASE REVERSAL

Output phase reversal occurs in some amplifiers when the input

common-mode voltage range is exceeded. As common-mode

voltage is moved outside the common-mode range, the outputs

of these amplifiers can suddenly jump in the opposite direction

to the supply rail. This is the result of the differential input pair

shutting down, causing a radical shifting of internal voltages

that results in the erratic output behavior.

The ADA4627-1 amplifier has been carefully designed to

prevent any output phase reversal if both inputs are maintained

within the specified input voltage range. If one or both inputs

exceed the input voltage range but remain within the supply

rails, an internal loop opens and the output varies. Therefore,

the inputs should always be a minimum of 3 V away from either

supply rail.

DRIVING CAPACITIVE LOADS

Adding capacitance to the output of any op amp results

in additional phase shift, which reduces stability and leads to

overshoot or oscillation. The ADA4627-1 has a high phase

margin and low output impedance, so it can drive reasonable

values of capacitance. This is a common situation when an

amplifier is used to drive the input of switched capacitor ADCs.

For other considerations and various circuit solutions, see the

Analog Dialogue article titled Ask the Applications Engineer-25,

Op Amps Driving Capacitive Loads, available at www.analog.com.