AD8337BCPZ-R2 datasheet(22/28 Pages) AD

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AD8337

Data Sheet

Rev. D | Page 22 of 28

The offset voltage effect of the AD8337, as with all VGAs, can

appear as a complex waveform when observed across the range

of VGAIN voltage. Generated by multiple sources, each device has

a unique offset voltage (VOS) profile while the GAIN input is

swept through its voltage range. The offset voltage profile seen

in Figure 15 is a typical example. If the VGAIN input voltage is

modulated, the output is the product of the VGAIN and the dc

profile of the offset voltage. This is observed on a scope as a

small ac signal, as shown in Figure 74. In Figure 74, the signal

applied to the VGAIN input is a 1 kHz ramp, and the output voltage

signal is slightly less than 4 mV p-p.

–4

–800

–2

VGAIN (mV)

–600

–200

–400

400

600

200

800

–10

–8

–6

VS = 2.5

INPUT

OUTPUT

VS = ±2.5V

Figure 74. Offset Voltage vs. VGAIN for a 1 kHz Ramp

The profile of the waveform shown in Figure 74 is consistent

over a wide range of signals from dc to about 20 kHz. Above

20 kHz, secondary artifacts can be generated due to the effects

of minor internal circuit tolerances, as shown in Figure 75.

These artifacts are caused by settling and time constants of the

interpolator circuit and appear at the output as the voltage

spikes, as shown in Figure 75.

–4

–800

–2

VGAIN (mV)

–600

–200

–400

400

600

200

800

–10

–8

–6

VS = 2.5

INPUT

OUTPUT

SPIKE

VS = ±2.5V

Figure 75. VOS Profile for a 50 kHz Ramp

Under certain circumstances, the product of VGAIN and the

offset profile plus spikes is a coherent spurious signal within the

signal band of interest and indistinguishable from desired

signals. In general, the slower the ramp applied to the GAIN

Pin, the smaller the spikes are. In most applications, these

effects are benign and not an issue.

THERMAL CONSIDERATIONS

The thermal performance of LFCSPs, such as the AD8337,

departs significantly from that of leaded devices such as the

larger TSSOP or QFSP. In larger packages, heat is conducted

away from the die by the path provided by the bond wires and

the device leads. In LFCSPs, the heat transfer mechanisms are

surface-to-air radiation from the top and side surfaces of the

package and conduction through the metal solder pad on the

mounting surface of the device.

θJC is the traditional thermal metric used for integrated circuits.

Heat transfer away from the die is a three-dimensional dynamic,

and the path is through the bond wires, leads, and the six

surfaces of the package. Because of the small size of LFCSPs, the

θJC is not measured conventionally. Instead, it is calculated using

thermodynamic rules.

The θJC value of the AD8837 listed in Table 2 assumes that the

tab is soldered to the board and that there are three additional

ground layers beneath the device connected by at least four vias.

For a device with an unsoldered pad, the θJC nearly doubles,

becoming 138°C/W.

PSI (Ψ)

Table 2 lists a subset of the classic theta specification, ΨJT (Psi

junction to top). θJC is the metric of heat transfer from the die to

the case, involving the six outside surfaces of the package. Ψ(XY)

is a subset of the theta value and the thermal gradient from the

junction (die) to each of the six surfaces. Ψ can be different for

each of the surfaces, but since the top of the package is a fraction of

a millimeter from the die, the surface temperature of the package is

very close to the die temperature. The die temperature is calculated

as the product of the power dissipation and ΨJT. Since the top

surface temperature and power dissipation are easily measured, it

follows that the die temperature is easily calculated. For example,

for a dissipation of 180 mW and a ΨJT of 5.3°C/W, the die

temperature is slightly less than 1°C higher than the surface

temperature.

BOARD LAYOUT

Because the AD8337 is a high frequency device, board layout is

critical. It is very important to have a good ground plane

connection to the VCOM pin. Coupling through the ground

plane, from the output to the input, can cause peaking at higher

frequencies.

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