REFERENCE CIRCUIT DESIGN

CLOCKING

REF

T

REF

B

I

SET

0.1

µF

2.2

µF

2

Ω

56.2k

Ω

2

Ω

2.2

µF

0.1

µF

ADS5277

ADS5277

SBAS333D – FEBRUARY 2005 – REVISED JANUARY 2009............................................................................................................................................ www.ti.com

The device also supports the use of external

reference voltages. This mode involves forcing REFT

The digital beam-forming algorithm relies on gain

and REFB externally and the internal reference buffer

matching across all receiver channels. A typical

is tri-stated. Since the switching current for the eight

system would have about 12 octal ADCs on the

ADCs come from the externally-forced references, it

board. In such a case, it is critical to ensure that the

is possible for the performance to be slightly less than

gain is matched, essentially requiring the reference

when the internal references are used. It should be

voltages seen by all the ADCs to be the same.

noted that in this mode, VCM and ISET continue to be

Matching references within the eight channels of a

generated from the internal bandgap voltage, as in

chip is done by using a single internal reference

the internal reference mode. It is therefore important

voltage buffer. Trimming the reference voltages on

to ensure that the common-mode voltage of the

each chip during production ensures the reference

externally-forced

reference

voltages

matches

to

voltages are well-matched across different chips.

within 50mV of VCM. The state of the reference

voltages during various combinations of PD and

All bias currents required for the internal operation of

INT/EXT is shown in Table 1.

the device are set using an external resistor to

ground at pin ISET. Using a 56.2kΩ resistor on ISET

Table 1. State of Reference Voltages for Various

generates an internal reference current of 20A. This

Combinations of PD and INT/EXT

current is mirrored internally to generate the bias

current for the internal blocks. Using a larger external

PD

0

0

1

1

resistor at ISET reduces the reference bias current,

INT/EXT

0

1

0

1

and thereby scales down the device operating power.

REFT

Tri-State

1.95V

Tri-State

Tri-State

However, it is recommended that the external resistor

REFB

Tri-State

0.95V

Tri-State

Tri-State

be within 10% of the specified value of 56.2k

Ω so

that the internal bias margins for the various blocks

VCM

1.45V

1.45V

Tri-State(1)

Tri-State(1)

are proper.

(1) Weak pull-down (approximately 5k

Ω) to ground.

Buffering the internal bandgap voltage also generates

a voltage called VCM, which is set to the midlevel of

REFT and REFB, and is accessible on a pin. It is

meant as a reference voltage to derive the input

The eight channels on the chip operate from a single

common-mode in case the input is directly coupled. It

ADCLK input. To ensure that the aperture delay and

can

also

be

used

to

derive

the

reference

jitter are same for all the channels, a clock tree

common-mode voltage in the external reference

network is used to generate individual sampling

mode.

clocks to each channel. The clock paths for all the

channels are matched from the source point all the

When using the internal reference mode, a 2

Ω

way to the sample-and-hold amplifier. This ensures

resistor should be added between the reference pins

that the performance and timing for all the channels

(REFT and REFB) and the decoupling capacitor, as

are identical. The use of the clock tree for matching

shown in Figure 36. If the device is used in the

introduces an aperture delay, which is defined as the

external reference mode, this 2

Ω resistor is not

delay between the rising edge of ADCLK and the

required.

actual instant of sampling. The aperture delays for all

the channels are matched to the best possible extent.

However, a mismatch of ±20ps (±3

σ) could exist

between the aperture instants of the eight ADCs

within the same chip. However, the aperture delays of

ADCs across two different chips can be several

hundred

picoseconds

apart.

Another

critical

specification is the aperture jitter that is defined as

the uncertainty of the sampling instant. The gates in

the clock path are designed to provide an rms jitter of

approximately 1ps.

Ideally the input ADCLK should have a 50% duty

cycle. However, while routing ADCLK to different

components on board, the duty cycle of the ADCLK

reaching the ADS5277 could deviate from 50%. A

smaller (or larger) duty cycle reduces the time

available for sample or hold phases of each circuit,

and is therefore not optimal. For this reason, the

Figure 36. Internal Reference

internal PLL is used to generate an internal clock that

22

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