ADA4627-1/ADA4637-1

Rev. C | Page 15 of 20

THD + N MEASUREMENTS

Total harmonic distortion plus noise (THD + N) is usually

measured with an audio analyzer, such as those from Audio

Precision, Inc™. The analyzer consists of a low distortion

oscillator that is swept from the starting frequency to the

ending frequency. The 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;

therefore, 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 49.

0.01

0.1

1

10

100

FREQUENCY (kHz)

80kHz FILTER

500kHz FILTER

0.01

0.001

0.0001

0.00001

ADA4627-1

Figure 49. 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/ADA4637-1 at room temperature, leakage paths

must be considered. A printed circuit board (PCB), with dust

and humidity, can have 100 MΩ of resistance over a few tenths

of an inch. A 1 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 50), the inverting and noninverting nodes can be

surrounded by traces held at a quiet analog ground.

2

3

6

8

ADA4627-1

+

–

GUARD

Figure 50. 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 51).

3

2

6

8

ADA4627-1

+

–

+

–

GUARD

Figure 51. 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 mm. 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/ADA4637-1 amplifiers have 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.