OPA835IDBVT datasheet(33/53 Pages) NSC

33 / 53 page

OPA 835

2 kW

OUT

100

f - Frequency - MHz

V = 5 V,

= 100 Vpp,

G = 2,

R = 2 k

OUT

-9

-6

-3

R = 100 k

R = 10 k

R = 2 k

R = 100 k

C = 1 pF

R = 10 k

C = 1 pF

OPA835

OPA2835

www.ti.com

SLOS713E – JANUARY 2011 – REVISED JULY 2013

Low Power Applications and the Effects of Resistor Values on Bandwidth

The OPA835 and OPA2835 are designed for the nominal value of RF to be 2 kΩ in gains other than +1. This

gives excellent distortion performance, maximum bandwidth, best flatness, and best pulse response. It also loads

the amplifier. For example; in gain of 2 with RF = RG = 2 kΩ, RG to ground, and VOUT = 4 V, 1mA of current will

flow through the feedback path to ground. In gain of +1, RG is open and no current will flow to ground. In low

power applications, it is desirable to reduce this current by increasing the gain setting resistors values. Using

larger value gain resistors has two primary side effects (other than lower power) due to their interaction with

parasitic circuit capacitance.

1. Lowers the bandwidth.

2. Lowers the phase margin

(a) This will cause peaking in the frequency response.

(b) And will cause over shoot and ringing in the pulse response.

Figure 69 shows the small signal frequency response on OPA835EVM for non-inverting gain of 2 with RF and RG

equal to 2 k

Ω, 10 kΩ, and 100 kΩ. The test was done with RL = 2 kΩ. Due to loading effects of RL, lower values

may reduce the peaking, but higher values will not have a significant effect.

Figure 69. Frequency Response with Various Gain Setting Resistor Values

As expected, larger value gain resistors cause lower bandwidth and peaking in the response (peaking in

frequency response is synonymous with overshoot and ringing in pulse response). Adding 1 pF capacitors in

parallel with RF helps compensate the phase margin and restores flat frequency response. Figure 70 shows the

test circuit used.

Figure 70. G=2 Test Circuit for Various Gain Setting Resistor Values

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