ADAU1977WBCPZ-R7 datasheet(19/68 Pages) AD

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Data Sheet

ADAU1977

Rev. C | Page 19 of 68

Line Inputs

This section describes some of the possible ways to connect the

ADAU1977 for line level inputs.

Line Input Balanced or Differential Input DC-Coupled Case

For example, in the case of a typical power amplifier for an auto-

mobile, the output can swing around 10 V rms differential with

approximately 7.2 V common-mode dc input voltage (assuming

a 14.4 V battery and bridge-tied load connection). The signal at

each input pin has a 5 V rms or 14.14 V p-p signal swing. With

a common-mode dc voltage of 7.2 V, the signal can swing between

(7.2 V + 7.07 V) = +14.27 V p-p and (7 V − 7.07 V) = 0.13 V at

each input. Therefore, this results in approximately a 28.54 V p-p

differential signal swing and measures around −0.16 dBFS (ac

only with dc high-pass filter) at the ADC output. See Figure 17.

Line Input Balanced or Differential Input AC-Coupled Case

For an amplifier output case with ac coupling, refer to Figure 18

for information about connecting the line level inputs to the

ADAU1977. In this case, the AINxP/AINxN pins must be

pulled up to the required common-mode level using the

resistors on MICBIAS. The VCM must be such that the input

never swings below a ground. In other words, if the input signal

is 14 V p-p, the VCM must be around 14 V/2 = 7 V to ensure that

the signal never swings below a ground. The microphone bias

can provide the required clean reference for generating the VCM.

The R1 value can be calculated as follows:

R1 = Rin1977 (MB − VCM)/VCM

where:

VCM is the peak-to-peak input swing divided by 2.

MB = 8.5 V.

Rin1977 is the single-ended input resistance (see Table 1).

However, in this case the equivalent input resistance of AINxP/

AINxN is reduced and can be calculated as R1 || Rin1977.

Input Resistance = R1 × Rin1977/(R1 + Rin1977)

where Rin1977 is the single-ended value from Table 1.

The C1 and C2 values can be determined for the required low

frequency cutoff using the following equation:

C1 or C2 = 1/(2 × π × fC × Input Resistance)

Line Input Unbalanced or Single-Ended Pseudo Differential

AC-Coupled Case

For a single-ended application, the signal swing is reduced by half

because only one input is used for the signal, and the other input is

connected to 0 V. As a result, the input signal capability is reduced

to 5 V rms in a single-ended application. With a common-mode dc

voltage of 7.2 V, the signal can swing between (7.2 V + 7.07 V)

= +14.27 V p-p and (7.2 V − 7 V) = 0.13 V. Therefore, this

results in approximately a 14.14 V p-p differential signal swing

and measures around −6.16 dBFS (ac only with dc high-pass

filter) at the ADC output. See Figure 19.

The values of the resistors (R1/R2) and capacitors (C1/C2) are

similar to those for the balanced ac-coupled case described in

the Line Input Balanced or Differential Input AC-Coupled Case

section.

Line Input Unbalanced or Single-Ended AC-Coupled Case

For a single-ended application, the signal swing is reduced by half

because only one input is used for the signal, and the other input is

connected to 0 V. As a result, the input signal capability is reduced

to 5 V rms in a single-ended application. With a common-mode dc

voltage of 7.2 V, the signal can swing between (7.2 V + 7.07 V) =

+14.27 V p-p and (7.2 V − 7 V) = 0.13 V. Therefore, this results

in approximately a 14.14 V p-p differential signal swing and

measures around −6.16 dBFS (ac only with dc high-pass filter)

at the ADC output. The difference in the common-mode dc

voltage between the positive and negative input (7.2 V) would

appear at the ADC output if the signal was not high-pass filtered.

See Figure 20.

The values of the resistor (R1) and capacitor (C1) are similar to

those for the balanced ac-coupled case described in the Line

Input Balanced or Differential Input AC-Coupled Case section.

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