AN-1384: Pairing A Driver Amplifier with the AD7768/AD7768-4 or the AD7768-1

Introduction

A common trend in analog input/output module design is increased channel count in a smaller form factor. This trend is driven by the need to reduce cost and testing time by increasing the number of measurements achievable from a single module or a peripheral component interconnect (PCI) extension for instrumentation (PXI) card slot. The increase in channel density contributes to an increase in thermal dissipation, which is a common issue for designers in modular applications. To design within the thermal budget requirements of high density data acquisition modules, customers must consider trade-offs for speed, bandwidth, and performance.

The AD7768/AD7768-4 and the AD7768-1 are 8-channiirel, 4-channel, and single-channel, 24-bit,simultaneous sampling analog-to-digital converters (ADCs). Selectable power modes and digital filter options reconfigure the AD7768/AD7768-4 and the AD7768-1 to suit a wide range of applications, such as industrial input/output modules, instrumentation, audio testing, control loops, and condition monitoring.

An external driver amplifier must drive the input to the AD7768/ AD7768-4 or the AD7768-1. Drive requirements for the analog front-end scale with the front-end sampling rate. Selectable precharge buffers on the AD7768/AD7768-4 or the AD7768-1 reduce the burden on front-end driver amplifiers, which allows lower power amplifiers to drive analog inputs with high sampling rates.

This application note outlines how to achieve −123.2 dB of total harmonic distortion (THD) at subsystem power levels as low as 13.25 mW per channel and compares combinations of high performance driver amplifiers and low power amplifiers with and without the assistance of the precharge buffers. These amplifiers are selected based on the suitability to drive the AD7768/AD7768-4 or the AD7768-1 in particular power modes for a fair and valid comparison. For example, the higher bandwidth amplifiersselected to drive the AD7768/AD7768-4 or the AD7768-1 in fast power mode work equally well in median or low power mode, but can burn more power than necessary depending on which system is being used. The appropriate combinations of driver amplifiers and power modes evaluated in this application note allow the design of a single data acquisition (DAQ) system platform to achieve the highest performance within specific bands of thermal or power constraints.

Connection Diagrams

Figure 1. Typical Connection Diagram for the AD7768/AD7768-4 in Fast Power Mode, with the ADA4945-1 as the Driver Amplifier.

Figure 2. Typical Connection Diagram for the AD7768-1in Fast Power Mode, with the ADA4945-1 as the Driver Amplifier.

Circuit Description

Amplifier testing for the AD7768/AD7768-4 and the AD7768-1 is conducted using various platforms, such as the EVALAD7768FMCZ, EVAL-AD7768-4FMCZ, EVAL-AD7768- 1FMCZ, and various amplifier mezzanine cards (AMCs). The amplifiers populated on each AMC are listed in the Amplifier Configuration section. The EVAL-AD7768FMCZ, EVAL-AD7768-4FMCZ, and EVAL-AD7768-1FMCZ evaluation platforms schematics are available for download on the respective evaluation board product pages on the Analog Devices, Inc., website. These evaluation platforms can be configured to use an AMC (on one channel only) as the driver amplifier input. See the EVAL-AD7768FMCZ, EVALAD7768-4FMCZ, and EVAL-AD7768-1FMCZ user guides for more details. The AMCs available are populated with various amplifiers and are designed specifically to work with Analog Devices ADCs. The EVAL-SDP-CH1Z is connected to the EVALAD7768FMCZ, EVAL-AD7768-4FMCZ, and EVAL-AD7768- 1FMCZ evaluation platforms to interface with the evaluation software, which is supplied with the evaluation hardware. A precision audio source is used for ac analysis. The following amplifiers selected for testing complement each of the different power modes on the AD7768/AD7768-4 or the AD7768-1:

In low power mode: ADA4805-2, ADA4841-2, ADA4945-1, and ADA4940-1.
In median mode: ADA4805-2, ADA4807-2, ADA4940-1, and ADA4945-1.
In fast power mode: ADA4807-2, ADA4896-2, ADA4899-1, ADA4940-1, and ADA4945-1.

Table 1 describes the performance and power specifications for the selected amplifiers. Some of these amplifiers are available in different package sizes and options.

Table 1. Amplifier Specifications
Amplifier	Features	Bandwidth (MHz	Slew (V/µs)	Voltage Noise Density (nV/√Hz)	Current Noise Density (pA/√Hz)	Offset Voltage (µV maximum)	Offset Drift (µV/°C)¹	Supply (V	Power per Amplifier (mA)
ADA4899-1	Unity gain, ultralow distortion	600	310	1	2.6	230	5	5 to 12	16
ADA4896-2	Low drift, rail-to-rail output (RRO)	230	120	1	2.8	500	0.2	±3 to ±5	3.0
ADA4807-2	Rail-to-rail input/ output (RRIO), low drift	180	225	3.1	0.7	125	0.7	±3 to ±5	1.0
ADA4805-2	RRO, low drif	105	160	5.9	0.6	125	0.2	±3 to ±5	0.625
ADA4940-1	RRO, differential amplifier	260	95	3.9	0.81	3.50	1.2	3 to 7	1.25 total2
ADA4841-2	RRIO low noise and distortion	80	13	2.1	1.4	300	1	2.7 to 12	1.2
ADA4084-2	RRIO, low power	13.9	3.7	3.9	0.55	300	0.5	±1.5 to ±15	0.625
ADA4945-1	RRO,selectable modes	80³ , 145⁴	100³ , 600⁴	3.0³ , 1.8⁴	0.6³ , 1.0⁴	±115	0.5	3 to 10	1.4³ total² , 4⁴ total ²

¹Values are typical unless otherwise noted.
²Total power for a single channel.
³Low power mode (see the ADA4945-1 data sheet).
⁴Full power mode (see the ADA4945-1 data sheet).

Selecting a Power Mode

The AD7768/AD7768-4 and the AD7768-1 have three selectable power modes: low, median, and fast. These power modes select an operating point for the AD7768/AD7768-4 or the AD7768-1 in which the optimal bandwidth and power consumption is chosen while maintaining the same dynamic range.

The power mode selected is used in conjunction with the master clock divider (MCLK_DIV) to set this operating point correctly. MCLK_DIV determines the frequency the modulator runs at. The modulator output is then decimated to give the final output data rate (ODR). The recommended modulator frequency (f_MOD) ranges are described in Table 2

Table 3 demonstrates that the power consumption scales with speed and bandwidth.

The two basic filter options available in the AD7768/ AD7768-4 and the AD7768-1 are the sinc filter or the finite impulse response (FIR) filter. Note that the FIR filters and the wideband filters used in the AD7768/AD7768-4 or the AD7768-1 have the same filter types, topology, and characteristics.

Analog Input Structure

Figure 3 shows the analog input structure of the AD7768/ AD7768-4 and the AD7768-1. The analog input precharge buffers are enabled on a per channel basis. When enabled, the analog input precharge buffers provide the charge to the sampling capacitors for the initial sampling period and charge the bulk of the current required to settle the sampling capacitors. The remaining charge is driven by the external amplifier, which charges the final finer settling of the sampling capacitors to achieve precision results.

The precharge buffers reduce the input current sourced from the amplifier from 320 µA to ~25 µA for a 5 V input at the fastest sampling rate. See the AD7768/AD7768-4 and the AD7768-1 data sheet for more information about the analog input structure. Figure 4 shows the AMC board and Figure 5 shows the ADA4807-2 AMC connected to the AD7768/AD7768-4 and the AD7768-1 evaluation board.

Table 2. Recommended f
Power Mode	Typical MCLK_DIV		Recommended f_MOD (MHz)
Power Mode	AD7768/AD7768-4	AD7768-1	Recommended f_MOD (MHz)
Low	MCLK/32	MCLK/16	0.0361 to 1.024
Median	MCLK/8	MCLK/4	1.024 to 4.096
Fast	MCLK/4	MCLK/2	4.096 to 8.192

¹Recommended fMOD is 0.038 MHz for the AD7768-1.

Table 3. Power Modes for the AD7768/AD7768-4 and the AD7768-1
Power Mode	Typical Speed (kSPS)	FIR2			Sinc Filter
		Bandwidth (kHz)	Power (mW)		Bandwidth (kHz)	Power (mW)
		Bandwidth (kHz)	AD7768/AD7768-4	AD7768-1	Bandwidth (kHz)	AD7768/AD7768-4	AD7768-1
Low	256	110.8	51.5	36.8	52.2	41	26.4
Median	128	55.4	27.5	19.7	26.1	22	14.4
Fast	32	13.8	9.375	6.75	6.5	8.5	5.4