EVAL-AD7760EB datasheet(20/37 Pages) AD

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AD7760

Rev. A | Page 19 of 36

MODULATOR DATA OUTPUT MODE

Operating the AD7760 in modulator output mode enables the

output of data directly from the Σ-Δ modulator. This mode of

operation bypasses the AD7760 on-board digital filtering

capabilities, outputting data in its unfiltered form.

As discussed in the Theory of Operation section, the AD7760

operates using oversampling, which spreads quantization noise

over a wide bandwidth. The decrease in the quantization noise

energy in the resulting signal band is illustrated in Figure 40a.

By coupling the use of oversampling with the use of a high

order, multibit Σ-Δ modulator, the AD7760 further reduces the

quantization noise in the signal band. Figure 41 is an FFT of

unfiltered data output from the AD7760 when it is used in

modulator output mode. This clearly demonstrates the shaping

of the quantization noise performed by the AD7760’s Σ-Δ

modulator.

MODULATOR INPUTS

The maximum voltage input to each differential modulator

input pin is 0.8 × 4.096 V ≈ 3.275 V (80% of VREF), which must

sit on a common mode of VREF/2. This maximum differential

input voltage is shown as the conditioned output of the AD7760’s

on-board differential amplifier in Figure 52 in the Driving the

AD7760 section.

Further details on the signal conditioning implemented by the

AD7760’s on-board differential amplifier and the recommended

external circuitry that accompanies it is described in the

Driving the AD7760 section.

–160

FREQUENCY (MHz)

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

–130

–140

–150

Figure 41. FFT of Data Output by the AD7760 in Modulator Output Mode

MODULATOR DATA OUTPUT SCALING

In modulator output mode, data is output in a 16-bit twos

complement format on Pins D [15:0]; however, this data is

scaled to 15 bits. The transfer function in Figure 42 shows the

scaling involved for the 16 data bits output from Modulator

Pins D[15:0] vs. the maximum differential voltage input

allowed for the modulator inputs (VIN+ and VIN−).

D[15:0]

0011 0011 0011 0010

0011 1111 1111 1111

0000 0000 0000 0000

1100 1100 1100 1100

1100 0000 0000 0000

+4.096V

–4.096V

VIN+ = 3.6855V

VIN– = 0.4105V

+3.275V = MODULATOR FULL SCALE = 80% OF +4.096V

80% OF +4.096V = MODULATOR FULL SCALE = –3.275V

VIN+ = 2.048V

VIN– = 2.048V

VIN+ = 0.4105V

VIN– = 3.6855V

Figure 42. Modulator Output Data Scaling

As the nature of the modulator output is coarse relative to

the fully filtered output of the AD7760 (due to the associated

quantization noise of the modulator output), Bits D[3:0] of the

modulator output are zero when operating in modulator data

output mode. Thus, the data outputs for the calculations listed

in Example 1 and Example 2 for inputs to the modulator pins

VIN+ and VIN− show Bits D[3:0] of the modulator output as zero.

Example 1

VIN+ = 3.5 V

VIN− = 0.595 V

Modulator Output Code = ([VIN(+) − VIN(−)]/4.096 V) × 16384

= [(3.5 V − 0.595 V)/4.096 V] × 16384

= +11620

Direct Scaling: [0010 1101 0110 0100]

Value Output on Data Output Pins D[15:0]:

D [15:0] = [0010 1101 0110 0000].

Example 2

VIN+ = 0.595 V

VIN− = 3.5 V

Modulator Output Code = ([VIN(+) − VIN(−)]/4.096 V) × 16384

= [(0.595 V − 3.5 V)/4.096 V] × 16384

= −11620

Direct Scaling: [1101 0010 1001 1100]

Value Output on Data Output Pins D[15:0]:

D [15:0] = [1101 0010 1001 0000].

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