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AD8330

Rev. D | Page 19 of 36

Input Common-Mode Range and Rejection Ratio

Input Pin INHI and Pin INLO should be ac-coupled in most

applications to achieve the stated noise performance. In general,

when direct coupling is used, care must be taken in setting the

dc voltage level at these inputs, and particularly when minimizing

noise is critical. This objective is complicated by the fact that

the common-mode level varies with the basic gain voltage, VDBS.

Figure 55 shows this relationship for a supply voltage of 5 V, for

temperatures of −40°C, +25°C, and +85°C. Figure 56 shows the

input noise spectral density (RS = 0) vs. the input common-

mode voltage, for VDBS = 0.5 V, 0.6 V, 0.75 V, and 1.5 V. It is

apparent that there is a broad range over which the noise is

unaffected by this dc level. The input CMRR is excellent (see

Figure 16).

VDBS (V)

2.6

3.2

3.1

3.0

2.9

2.8

2.7

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

T = +25°C

T = +85°C

T = –40°C

Figure 55. Common-Mode Voltage at Input Pins vs. VDBS, for VS = 5 V,

T = −40°C, + 25°C, and + 85°C

COMMON-MODE VOLTAGE AT INHI, INLO (V)

0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.8

VDBS = 1.5V

VDBS = 0.75V

VDBS = 0.6V

VDBS = 0.5V

SIMULATION

Figure 56. Input Noise vs. Common-Mode Input Voltage for

VDBS = 0.5 V, 0.6 V, 0.75 V, and 1.5 V

Output Noise and Peak Swing

The output noise of the AD8330 is the input noise multiplied by

the overall gain, including any optional change to the voltage,

VMAG, applied to Pin VMAG. The peak output swing is also

proportional to this voltage, which, at low gains and high values

of VMAG, affects the output noise.

The scaling for VDBS = 0 V is as follows:

VOUT_PK = ±4 VMAG

(8)

VNOISE_OUT = (85 + 70 VMAG) nV/√Hz

(9)

For example, using a reduced value of VMAG = 0.25 V that lowers

all gain values by 6 dB, the peak output swing is ±1 V (differ-

entially) and the output noise spectral density evaluates to

102.5 nV/√Hz. The peak output swing is no different at full

gain, but the noise becomes

VNOISE_OUT = (0.1 + 0.32 VMAG) μV/√Hz

(10)

for RS = 0 and VDBS = 1.5 V, assuming an input noise of 5 nV/√Hz.

The output noise for very small values of VMAG (at or below 15 mV)

is not precise, partly because the small input offset associated

with this interface has a large effect on the gain.

Offset Compensation

The AD8330 includes an offset compensation feature that is

operational in the default condition (no connection to Pin OFST).

This loop introduces a high-pass filter function into the signal

path, whose −3 dB corner frequency is at

(

)

INT

HPF

(11)

where:

CHP is the external capacitance added from OFST to CNTR.

RINT is an internal resistance of approximately 480 Ω, having a

maximum uncertainty of about ±20%.

This evaluates to

HPF

330

(CHP in μF)

(12)

A small amount of peaking at this corner when using small

capacitor values can be avoided by adding a series resistor.

Useful combinations are CHP = 3 nF, RHP = 180 Ω, f = 100 kHz;

CHP = 33 nF, RHP = 10 Ω, f = 10 kHz; CHP = 0.33 μF, RHP = 0 Ω,

f = 1 kHz; CHP = 3.3 μF, RHP = 0 Ω, f = 100 Hz.

The offset compensation feature can be disabled simply by

grounding the OFST pin. This provides a dc-coupled signal

path, with no other effects on the overall ac response. Input

offsets must be externally nulled in this mode of operation, as

shown in Figure 58.

Effects of Loading on Gain and AC Response

The differential output impedance (RO) is 150 Ω, and the fre-

quency response of the output stage is optimized for operation

with a certain load capacitance on each output pin (OPHI and

OPLO) to ground, in combination with a load resistance (RL)

directly across these pins. In the absence of these capacitances,

there is a small amount of peaking at the top extremity of the

ac response. Suitable combinations are: RL = ∞, CL = 12 pF;

RL = 150 Ω, CL = 25 pF; RL = 75 Ω, CL = 40 pF; or RL = 50 Ω,

CL = 50 pF.

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