9

Maxim Integrated

Dual 36V Op Amp for 18-Bit

SAR ADC Front-End

MAX9633

Detailed Description

The MAX9633 is designed in a new 36V, high-speed

complementary BiCMOS process that is optimized for

excellent AC dynamic performance combined with high-

voltage operation.

The exceptionally fast settling time, low noise, low distor-

tion, high bandwidth, and low input offset voltage make

the IC an excellent solution to drive (up to 18-bit)high-

resolution and fast SAR ADCs.

The MAX9633 is unity gain stable and operates either

with a single supply voltage up to 36V or with dual sup-

plies up to Q18V.

Applications Information

Driving High-Resolution SAR ADCs

High-resolution SAR ADCs typically switch an input

capacitor in the order of tens of pF during the track and

hold phases. Such capacitor switching can cause a

voltage glitch at the input of the ADC that behaves as a

load-transient condition for the driving amplifier. In many

applications, this glitch is avoided by placing an external

capacitor at the ADC input that is in the order of 20 to 50

times the ADC input capacitor. If the ADC input capaci-

tor ranges from 15pF to 30pF, then the external capacitor

is anything between 300pF to 1.5nF, depending on the

application. An isolation resistor can be placed in series

between the amplifier’s output and the external capaci-

tor, as shown in the Typical Application Circuit.

During the load-transient condition described, the driv-

ing amplifier must be able to settle to 0.5 x LSB within

approximation, the number of time constants required to

settle to 0.5 x LSB is a logarithm function of the number

N of bits:

( )

N1

1)

=

The external RC time constant must be such that:

2)

As an example, consider a 16-bit SAR ADC with 500ns

acquisition time and 20pF input capacitor.

From 1):

k = 12

Assuming a factor of 50 for the external capacitor:

C = 1nF

The IC is optimized for very fast load-transient recovery

with big capacitive loads and small isolation resistors.

This makes it ideal to drive high-resolution and fast SAR

ADCs.

Recommended SAR ADCs

The MAX9633’s wide supply range and fast settling

make it ideal for driving high-resolution SAR ADCs, such

as the MAX1320. The MAX1320 is a 14-bit, 8-chan-

nel, simultaneous-sampling ADC that measures analog

inputs up to Q5V. Sampling up to 250ksps per channel

for eight channels, the MAX1320 achieves 77dB SNR,

90dBc SFDR, and -86dB THD. The MAX1320’s fast

sample rate and typical input resistance of 8.6kI often

make it necessary to have a low-noise op amp, such as

the MAX9633, driving its inputs. The MAX9633 is also

a good fit for an anti-aliasing active filter prior to the

MAX1320 as shown in the Typical Application Circuit.

The MAX1320 is part of a family of simultaneous sampling

ADCs (MAX1316–MAX1326). Other options include ADCs

that measure 0V to 5V inputs, or Q10V inputs, and two 4

or 8 simultaneous input channels. The MAX1320’s high

speed and resolution make it a fit for multiphase motor

control and power-grid monitoring.

The MAX9633 is also well-suited to drive the 16-bit

MAX11046 8-channel, simultaneous-sampling, SAR

ADC. The MAX11046 is rated for up to 250ksps. An input

driver is typically not necessary at sampling rates below

100ksps. For applications that require > 100ksps sample

rates, the MAX9633 offers small size, high bandwidth,

and ultra-low -100dB THD at 100kHz.

Low Noise and Low Distortion

The MAX9633 is designed for applications that require

very low voltage noise, making it ideal for low source

impedance. When driving 16-bit SAR ADCs with a Q5V

full-scale input, such as the MAX11046, the MAX9633

very low input voltage noise density specification guaran-

tees 16-bit resolution up to 10MHz of signal bandwidth.

The MAX9633 is also designed for ultra-low distor-

tion performance. THD specifications in the Electrical

Characteristics and Typical Operating Characteristics

is calculated up to the 5th harmonic. Even when driving

excellent low distortion operation up and beyond 100kHz

of bandwidth.

Besides driving high-resolution and high-bandwidth SAR

ADCs, applications that benefit for low-noise and low-

distortion applications can be found in industrial power-

grid and smart-grid, industrial motor-control, medical

imaging, automated test equipment, instrumentation,

and professional audio equipment.