SSM2143

REV.

–7–

APPLICATIONS INFORMATION

The SSM2143 is designed as a balanced differential line re-

ceiver. It uses a high speed, low noise audio amplifier with four

precision thin-film resistors to maintain excellent common-mode

rejection and ultralow THD. Figure 25 shows the basic differen-

tial receiver application where the SSM2143 yields a gain of 1/2.

The placement of the input and feedback resistors can be

switched to achieve a gain of +2, as shown in Figure 26. For

either circuit configuration, the SSM2143 can also be used un-

balanced by grounding one of the inputs. In applications requir-

ing a gain of +1, use the SSM2141.

6

3

5

1

4

2

7

6k

6k

12k

12k

–IN

+IN

–15V

0.1µF

+15V

0.1µF

A = 2

V

SSM2143

6

1

2

4

5

7

12k

12k

6k

6k

–IN

+IN

–15V

0.1µF

+15V

0.1µF

A =

V

1

2

SSM2143

3

+

Figure 25. Standard Config-

uration for Gain of 1/2

CMRR

The internal thin-film resistors are precisely trimmed to achieve

a CMRR of 90 dB. Any imbalances introduced by the external

circuitry will cause a significant reduction in the overall CMRR

performance. For example, a 5

Ω source imbalance will result in

a CMRR of 71 dB at dc. This is also true for any reactive source

impedances that may affect the CMRR over the audio frequency

range. These error sources need to be minimized to maintain

the excellent CMRR.

To quantify the required accuracy of the thin film resistor

matching, the source of CMRR error can be analyzed. A resistor

mismatch can be modelled as shown in Figure 27. By assuming

a tolerance on one of the 12 k

Ω resistors of ∆R, the equation for

the common-mode gain becomes:

=

6k

6k

+12k

6k

12k

+∆R

+1









–

6k

12k

+∆R

which reduces to:

=

1/3

∆R

12k

+∆R

This gain error leads to a common-mode rejection ratio of:

CMRR

=

≅

18k

∆R

–IN

+IN

6k

6k

12k +

∆R

12k

CMRR =

18k

∆R

Figure 27. A Small Mismatch in Resistance Results in a

Large Common-Mode Error

Setting

∆R to 5 Ω results in the CMRR of 71 dB, as stated

above. To achieve the SSM2143’s CMRR of 90 dB, the resistor

mismatch can be at most 0.57

Ω. In other words, to build this

circuit discretely, the resistors would have to be matched to

better than 0.005%!

The following table shows typical resistor accuracies and the

resulting CMRR for a differential amplifier.

% Mismatch

CMRR

5%

30 dB

1%

44 dB

0.1%

64 dB

0.005%

90 dB

DC OUTPUT LEVEL ADJUST

The reference node of the SSM2143 is normally connected to

ground. However, it can be used to null out any dc offsets in

the system or to introduce a dc reference level other than

ground. As shown in Figure 28, the reference node needs to be

REFERENCE

OP27

+10V

–10V

6

1

2

3

4

5

7

12k

12k

6k

6k

–IN

+IN

–15V

0.1µF

+15V

0.1µF

SSM2143

Figure 28. A Low Impedance Buffer Is Required to Adjust

the Reference Voltage.

buffered with an op amp to maintain very low impedance to

achieve high CMRR. The same reasoning as above applies such

that the 6 k

Ω resistor has to be matched to better than 0.005%

or 0.3

Ω. The op amp maintains very low output impedance

over the entire audio frequency range, as long as its bandwidth

is well above 20 kHz. The reference input can be adjusted over

a

±10 V range. The gain from the reference to the output is

unity so the resulting dc output adjustment range is also

±10 V.

INPUT ERRORS

The main dc input offset error specified for the SSM2143 is the

Input Offset Voltage. The Input Bias Current and Input Offset

Current are not specified as for a normal operational amplifier.

Because the SSM2143 has built-in resistors, any bias current

related errors are converted into offset voltage errors. Thus, the

offset voltage specification is a combination of the amplifier’s

offset voltage plus its offset current times the input impedance.

Figure 26. Reversing the

Resistors Results in a

Gain of 2

Figure 29. SSM2142/SSM2143 Balanced Line Driver/

Receiver System

2

6

5

4

3

8

7

SSM2142

SSM2143

+18V

–18V

+18V

0.1µF

0.1µF

–18V

3

7

6

4

1

2

5

ALL CABLE MEASUREMENTS USE

BELDEN CABLE (500').

1

A