Table 8. 1-kHz AC Analysis: Test Circuit Versus PCM4204 Data Sheet Typical Specifications
(F_S = 48 kSPS)

Table 9. THS4521EVM Parts List

9.2.1.2.1 Audio ADC Driver Performance: THS4521 and PCM3168 Combined Performance

The THS4521 is also tested as the drive amplifier for the PCM3168A ADC input. The PCM3168A is a high-performance, single-chip, 24-bit, 6-in/8-out, audio coder/decoder (codec) with single-ended and differential selectable analog inputs and differential outputs. The six-channel, 24-bit ADC employs a ΔΣ modulator and supports 8-kHz to 96-kHz sampling rates and a 16-bit/24-bit width digital audio output word on the audio interface. The eight-channel, 24-bit digital-to-analog converter (DAC) employs a ΔΣ modulator and supports 8-kHz to 192-kHz sampling rates and a 16-bit/24-bit width digital audio input word on the audio interface. Each audio interface supports I2S™, left-/right-justified, and DSP formats with 16-bit/24-bit word width. In addition, the PCM3168A supports the time-division-multiplexed (TDM) format.. The PCM3168A provides flexible serial port interface and many other advanced features. Refer to the PCM3168A product data sheet for more information.

The PCM3168A EVM is used to test the audio performance of the THS4521 as a drive amplifier. The standard PCM3168A EVM is provided with OPA2134 operational amplifiers that are used to convert single-ended inputs to differential to drive the ADC. For testing, the operational amplifier output series resistors are removed from one of the channels and a THS4521, mounted on its standard EVM, is connected to the ADC inputs via short coaxial cables. The THS4521 EVM is configured for both differential inputs as shown in Figure 91 and for single-ended input as shown in Figure 92 with 1-kΩ resistors for R_F and R_G, and 24.9-Ω resistors in series with each output to isolate the outputs from the reactive load of the coaxial cables. To limit the noise from the external EVM and cables, a 2.7-nF capacitor is placed differentially across the PCM3168A inputs. The THS4521 is operated with a single-supply +5-V supply so the output common-mode of the THS4521 defaults to +2.5 V as required at the input of the PCM3168A. The PCM3168A EVM is configured and operated as described in the PCM3168AEVM User's Guide. The ADC was tested with an external THS4521 EVM with both single-ended input and differential inputs. In both configurations, the results are the same. Figure 81 shows the THD+N versus frequency and Table 10 compares the result to the PCM3168 data sheet typical specification at 1 kHz. Both graphs show that it makes an excellent drive amplifier for this ADC. Note: a 2700 series Audio Analyzer from Audio Precision is used to generate the input signals to the THS4521 and to analyze the digital data from the PCM3168.

Figure 81. THS4521 and PCM3168: Thd+N Versus Frequency With No Weighting

Table 10. 1-kHz AC Analysis: Test Circuit vs PCM3168 Data Sheet Typical Specifications (F_S = 48 kSPS)

Configuration	Tone	THD+N
THS4521 and PCM3168	1 kHz	–92.6 dBc
PCM3168A Data sheet (typ)	1 kHz	–93 dBc

Table 11. AC Analysis

9.2.2.2.1 ADC Driver Performance: THS4521 and ADS8321 Combined Performance

To demonstrate achievable performance, the THS4521 is tested as the drive amplifier for the ADS8321 16-bit SAR ADC. The ADS8321 offers excellent ac and dc performance, with ultra-low power and small size. The circuit shown in Figure 86 was used to test the performance.

Data were taken using the ADS8321 at 100 kSPS with input frequencies of 2 kHz and 10 kHz and signal levels that were -0.5 dBFS. FFT plots that illustrate the spectral performance are given in Figure 89 and Figure 90. Tabulated ac analysis results are listed in Table 12 and compared to the ADS8321 data sheet typical performance. Note the significant improvement in SFD using the THS4521 driver over just the ADC by itself.

Figure 86. THS4521 and ADS8321 Test Circuit